• The true cause of implant failure is not the fault of the device but in fact the failure of the surgeon to understand the principles of fixation and limitations of the chosen implant.
• Wear, implant failure and osteolysis is classic basic science material that orthopaedic surgeons require a depth of knowledge and understanding.

• Wear can either be mechanical or corrosive.
• Mechanical wear is defined as the volume of material removed as a consequence of relative motion between two surfaces.
• It is dependent on the load applied and the sliding distance.

• The three main types of wear are abrasive, adhesive and fatigue.

Abrasive wear

• This occurs between two surfaces with different levels of hardness. The harder surface ploughs through the softer surface.

Adhesive wear

• This occurs when two materials in contact form a weld that fractures off from one of the surfaces during continued motion and transfers onto the second surface.

Fatigue wear

• This occurs when surface and subsurface cyclic shear stresses or strains in the softer material of an articulation exceed the fatigue limit for the material.¹
• Adhesive, abrasive and fatigue wear occur in both PE acetabular hip and tibial knee components. Adhesive and abrasive wear dominates in PE acetabular hip components, whereas fatigue wear is a very important wear mechanism in PE tibial knee components.
• Other types of wear include fretting wear, third body, erosive and corrosive wear.

Fretting wear

• This occurs due to repeated cyclical rubbing between two solid surfaces in contact. This type of material loss is often associated with a combination of adhesive and abrasive wear.

Erosive wear

• This is a material loss from the abrasive action of moving fluids on a solid surface.
• Occurs typically with a coupling in which one of the agonists is in the fluid stage and is associated with the abrasion produced by particles contained in the fluid.
• This can be quite dominant with orthopaedic implants because they are in direct contact with the body fluid environment. The major difference between abrasive and erosive wear is that erosion involves transfer of kinetic energy to the surface while abrasive wear does not.

Corrosive wear

• This is wear due to chemical or electrochemical reaction and unlike other forms of wear is not dependent on loading and sliding distance. It is the reason its not ideal to mix material in plates and screws.

Third-body wear

• The mechanism of third body wear is usually abrasive. It occurs when hard particles become embedded in a soft tissue.

• There are four modes of wear:

1. Mode 1 is the generation of wear debris that occurs with motion between the two bearing surfaces as intended by the designers.
2. Mode 2 refers to a primary bearing surface rubbing against a secondary surface in a manner not intended by the designers. Examples include a femoral head articulating with an acetabular shell following wear-through of the polyethylene.
3. Mode 3 refers to two primary bearing surfaces with interposed third-body particles (such as bone, cement, metal, ceramic, hydroxyapatite and so on).
4. Mode 4 refers to two non-bearing surfaces rubbing together (such as back-sided wear of an acetabular liner, fretting of the Morse taper, stem-cement fretting.

• While several modes of wear often occur simultaneously, mode 1 accounts for the majority of wear in well functioning hip or knee replacements.

• Inert
• Non-toxic
• Cheap
• Give good pain relief
• Be well tolerated without significant side effects
• Allow return to reasonable level of activity
• Last a reasonable length of time

• Stainless steel
• Cobalt–chromium alloys
• Titatium alloys

• Chromium produces a protective self-regenerating chromium oxide layer that protects against corrosion.
• Molybdenum decreases the rate of slow passive dissolution of chromium oxide layer by up to 1000 times.
• Nickel imparts further corrosion resistance.
• Although stainless steel alloy is a strong, stiff, and biocompatible material it has slow and finite corrosion rate.
• The stainless steel alloy most commonly used for implants is 316 L (Figure 1). This is derived from the commonly used 18-8 stainless steel alloy (18% chromium, 8% nickel) used in tableware. The addition of molybdenum (3%) to the 18-8 alloy and the reduction of carbon content (0.03% max) confers improved corrosion resistance particularly to pitting and intergranular attack. Nickel is added.

Figure 1. Composition of 316L (3% molybd, 16% nickel & L = Low carbon content) stainless steel.Contains

• Iron (62.97%)
• Chromium (18%)
• Nickel (16%)
• Molybdenum (3%)
• Carbon (0.03%) 

• This is a highly reactive metal that rapidly gets coated with an oxide layer making it physiologically inert and resistant to most chemicals.
• Titanium has elastic modulus of approximately half that of stainless steel and cobalt chromium alloy.
• Titanium plates are used in trauma surgery as their lower stiffness reduces the severity of stress shielding and cortical osteoporosis.
• It is less prone to fatigue failure than stainless steel. Although is highly notch sensitive so care should be taken not to mark with clamps while positioning.
• Elastic modulus of stainless steel is 12 times that of cortical bone while the elastic modulus of titanium is six times that of cortical bone.

Agins HJ, Alcock NW, Bansal M, et al. Metallic wear in failed titanium-alloy total hip replacements: a histological and quantitative analysis. J Bone Joint Surg Am 1988; 70: 347–356.
Hospital for Special Surgery and Memorial-Sloan-Kettering Cancer Center, New York City.

• Agins et al. raised the problem of the clinical significance of black metallic wear debris deposited in the periarticualr tissues surrounding titanium alloy prostheses.
• The authors undertook a quantitative and histological analysis of the tissues around failed Ti-Al-V total hip arthroplasties (THA). They demonstrated that when a titanium THA failed for whatever reason, copious metallic debris can be generated that can be locally irritating and possibly toxic to the surrounding tissues. The authors postulated that this might contribute to implant loosening.
• The paper reported serious early failures of titanium implants, associated with severe metallosis. The suitability of titanium alloy as a material for prosthetic implants was called into question. The paper highlighted titanium’s susceptibility to fretting, corrosion and wear.
• There were several reasons why titanium was first considered for use as a material for THA. A number of stainless steel Charnley first generation femoral stem fractures had been reported in the literature. Charnley himself reported an incidence of 0.26% while other studies had suggested a value as high as 4.3%.³ In addition, there was the concern of femoral calcar resorption occurring due to proximal femoral stress shielding. Hypotheses at the time focused on altered stresses in the bone as a cause of mechanical failure rather than polyethylene debris induced osteolysis.
• By the mid-1970s the mechanical properties of titanium were known from its use in fracture healing. The use of titanium alloy prosthesis with its lower modulus of elasticity than stainless steel or cobalt-chrome would hopefully lead to reduction in femoral cortex resorption observed with these stiffer metals and thereby lessen the chance of stem fracture.
• Laboratory and clinical studies on the mechanical and wear properties of titanium alloy THA were very encouraging. The wear properties against polyethylene appeared to be superior to that of stainless steel and cobalt chrome alloy.
• Titanium implants have excellent resistance to corrosion and excellent biocompatibility properties. Due to its superior fatigue strength, various manufacturers began using titanium alloys for the femoral stems of THA in preference to stainless steel and cobalt-chrome alloys that had been in use for over 20 years.
• The Hospital for Special Surgery in New York had developed a titanium alloy THA for clinical use but surgeons were seeing early failures with their implant. These serious early failures were reported by Agins et al. who examined the histological features of the soft tissues adjacent to failed components.
• Early failures of titanium alloys have seen titanium fall out of favour as a material for cemented femoral stems. Titanium is no longer used as a material for femoral heads due to its poor wear characteristics. Extensive burnishing and scratching of titanium alloy femoral heads have been reported along with a ten-fold increase in wear rate.⁴
• Femoral stems made of titanium alloys are widely used for uncemented THA, with high survival and good long term results reported.

• Principle of combined action:
• Any material containing multiple phases which demonstrates properties of the constituent phases, such that an optimal combination of properties is obtained.
• The phases within a composite must be chemically distinct and have a clear interface.

Natural composites

• Wood is a combination of flexible cellulose and stiffer lignin.
• Bone is composed of the hard but brittle apatite and the tough collagen.

Matrix and dispersed phase

• These are the names given to the two phases of a composite the matrix is continuous and surrounds the dispersed phase.

Figure 2. Matrix and dispersed phase composites 

Properties

• The properties of a composite are a function of the phases with the following all influencing the final material:
• Relative amounts
• Particle size
• Particle distribution
• Particle orientation

Classification

• Particle reinforced
• Fibre reinforced
• Structural composites

Particle reinforced

• Further divided in large particles and dispersed. In both the composite is loaded through the matrix and a proportion is transferred to the dispersed phase. This relies on strong bonding between the phases. The dispersed phase also acts to limit the motion of dislocations.
• For large particle composties the equation below will give the upper (u) and lower (l) bound elastic modulus (Ec):

where

Fibre reinforced

• Most common type; Kevlar and carbon fibre fall into this group.
• They aim to have high strength and stiffness for weight.
• Material characteristics usually given in:
• Specific strength

ratio of tensile strength to specific gravity

• Specific modulus

ratio of modulus elasticity to specific gravity

• Fibre orientation can lie within two extremes either totally random or parallel alignment.
• Overall material properties are better when fibres are aligned.
• Aligned fibre composites are highly anisotropic.
• Equations for the elastic modulus are as follows:

In the fibre direction

At 90° to fibre direction

where

Structural composites

• Combinations of single materials and composites. Divided into two types laminar and sandwich.
• Laminar
• Composed of 2D sheets with a highly directional strength. These are then stacked and glued together varying the high strength direction.

• Sandwich
• These are considered as composites and consist of two sheets of stiff material bonded to a lightweight inner core. The outer sheets give resistance to the tensile and compressive loads while the inner core must be able to withstand the shear forces and buckling.

Figure 4. Sandwich composite 

• The polyethylene granules or powder; the raw source polymer.
• Medical grade polyethylene is a very small percentage of the worldwide production of polyethylene. Only ultra-high molecular weight material is used in the manufacturing of components for total joint replacement.

• A compound mixed with the polyethylene powder (in some grades of the material) before it is formed into a solid. The calcium stearate serves as a scavenger of residual acid. In ram extrusion, it also acts as a lubricant, and has been shown to help prolong the life of the manufacturing equipment. It also results in the polyethylene having a whiter colour. However, reports have indicated that fusion defects are more common in polyethylene that contains calcium stearate. Fusion defects may make the component more susceptible to crack initiation and propagation. Consequently, most manufacturers now use grades of polyethylene that do not contain calcium stearate. However, the quantitative effects of calcium stearate on the wear properties of polyethylene components are a subject of ongoing debate. In several retrieval studies, components manufactured from 1900 PE resin have shown significantly lower levels of oxidation following sterilisation by gamma irradiation in air. The reason(s) for this have not been clearly identified.

• Breakage of the long chains of polyethylene into shorter molecules. Extensive chain scission can substantially increase the crystallinity, density, stiffness and brittleness of the polyethylene, weakening the material. Oxidation is a primary cause of chain scission in polyethylene.

• A consolidation method that subjects the polyethylene powder to high temperature and pressure, fusing it into a solid form, either into bulk stock for subsequent machining, or into net-shape components.
• Consolidation – The fusing of polyethylene powder into a solid form by application of heat and pressure. The two principal methods of consolidation are compression molding and ram extrusion.
• Cross-shear – The particular type of stress applied to the surface of the polyethylene component due to the crossing-path motion of the femoral ball. Crossing-path motion is also present, albeit to a lesser extent, in some designs of knee prostheses. Studies have shown that polyethylene wear is 10 to 100 times greater with crossing path motion than with simple linear reciprocating motion.

• The process by which chemical bonds link carbon atoms in adjacent polyethylene molecules by combining two free radicals. Crosslinking has been shown in laboratory wear simulators (both hip and knee) markedly to improve the wear resistance of polyethylene.
• Electron beam irradiation – Also known as E-beam, the polyethylene is bombarded with high-energy electrons which induce crosslinking. Because there is more attenuation of an electron beam than gamma rays, a high beam energy (e.g. 10 MeV) is used to produce crosslinking in the polyethylene. Residual free radicals generated by the electron beam can be extinguished by an appropriate post-crosslinking thermal treatment to avoid long-term oxidative degradation.
• EtO sterilisation - A sterilisation method that utilises ethylene oxide gas (EtO). EtO does not induce free radicals or oxidation; it also does not induce crosslinking. To eliminate the toxic gas, components must be outgassed for a sufficient period prior to being implanted.
• Free radical – An electron on an atom that is a potential reaction site for oxidation or crosslinking.
• Gamma irradiation – Irradiation by exposure to a radioactive cobalt source, which emits gamma rays. Gamma radiation (in air) has been the predominant method used to sterilise prosthetic joint components for more than two decades, with free radical production, oxidation and crosslinking being unintentional byproducts. Only recently has gamma radiation been used intentionally to crosslink polyethylene to improve its wear resistance.
• Gas plasma sterilisation – A non-irradiation sterilisation method in which a device is exposed to energised oxygen, nitrogen and argon gas particles and a peracetic acid gas in alternating cycles. The plasma sterilises the product by inactivating microorganisms. As with EtO sterilisation, gas plasma does not generate free radicals, induce oxidation or crosslinking.
• Inert gas packaging – Sealing the polyethylene component in a package flushed with an inert gas, such as argon or nitrogen, to remove the oxygen present during sterilisation and subsequent shelf storage.
• Isostatic molding – A multi-step process that begins with the manufacture of a cylindrical compact of UHMWPE powder from which most of the air has been mechanically expelled. Subsequently, the compacted rods are sintered in a hot isostatic pressure (HIP) furnace in an argon-filled pouch to minimise oxidative degradation of the UHMWPE. The resulting rod stock may be considered to be a compression-molded form of that resin. Finished implants are then made either by turning or milling operations.
• Oxygenless packaging – Polyethylene components are sealed in a package in an atmosphere with minimal oxygen during sterilisation and subsequent shelf storage. Current versions include combinations of inert gas partial vacuum, and enclosing a packet containing an oxygen scavenger.

• Reaction of an oxygen molecule with a free radical on the polyethylene molecule. This typically leads to chain scission, indirectly increasing the crystallinity, density and stiffness of the polymer and reducing its resistance to fracture and wear.
• Ram extrusion – A consolidation process in which a ram is used to force the polyethylene powder through a heated nozzle, resulting in a fused bar as large as 6 inches in diameter. Careful control of the processing variables (principally, the extrusion rate and the nozzle temperature) is required to produce a fully and uniformly consolidated material (e.g. containing minimal fusion defects).

• Heating the polyethylene to neutralise residual free radicals and, thereby, stabilise it against long-term oxidation. Bulk polyethylene (molded blocks or extruded bars) can be heated above the melting temperature (about 150°C), held there for a number of hours, and then cooled (“remelting”) to extinguish the free radicals before being machined into a final component.
• In contrast, if radiation crosslinking is applied to the finished components, they cannot be remelted, but they can be heated to below the melt temperature and held at that temperature for a number of days (“annealing”) substantially to reduce the residual free radicals. Because annealing occurs with the polyethylene still in a semi-crystalline state (i.e. below the melting temperature), it is not as effective as remelting in eliminating the residual free radicals.

• Typically, the polyethylene components are placed in a barrier package, flushed with an inert gas (i.e. nitrogen) and then evacuated to minimise the oxygen present during radiation, sterilisation and subsequent shelf storage.

• produced at high temperatures.
• Interatomic bonding of ceramics ranges from purely ionic to purely covalent.
• Very high hardness values.
• Due to the brittle nature of ceramics they fracture at around 0.1% strain, therefore transverse bending tests are the standard rather than tensile testing.
• Ceramics have much higher fracture strengths in compression compared to tension.
• This is due to micro cracks inherent in the material. Stress is amplified in tension but not in compression.
• Ceramics demonstrate a brittle fracture pattern. As a crack propagates through a ceramic, at a critical fracture velocity branching occurs leading to shattering.

• 80% aluminium oxide
• 20% zirconium oxide
• New generations of ceramics have been designed to resist fracture propagation to avoid shatter. This is achieved by reducing the grain size and producing uniform crystals. The introduction of zirconium oxide crystals introduces areas of high stain that act to block crack propagation.

• The term implant failure suggests that the failed implant was inadequate for the function expected of it.
• Clinically, implant failure may be defined as a failure of the implantation procedure to produce satisfactory results.

• Forging
• Casting
• Milling
• Broaching
• Additive manufacture

• Causes of such failure can be grouped into four categories:

1. Surgical
2. Material
3. Idiosyncratic
4. Patient compliance

• Surgical failures relate to errors in surgical judgement or application technique including surgically introduced complications such as infection.
• Minimise the risk of surgically related implant failure by acquiring a solid understanding of both the mechanics of fracture fixation as well as the materials limitations of the devices implanted.

• Material failures may result from deficiencies in engineering design, manufacturing processing, or handling in the operating room.
• Clinically, material failure modes fall into three categories: (1) purely mechanical, (2) purely environmental, and (3) conjoint mechanical-environmental (Table 1).
• Purely mechanical failures are the result of direct overload including significant plastic deformation or fracture. A sudden load applied to an implant, for example when a patient falls, may produce an implant fracture. Although the loads transmitted through the musculoskeletal system can be high, direct overload should not be an issue with orthopaedic implants. A suitably designed implant should be capable of withstanding these stresses under normal circumstances in the vast majority of implants. In some implants that have failed there have been through design features or surgical errors stresses that have been raised in the vicinity of the affected area. The material has failed at stresses lower than the yield point (fatigue failure).
• Environmental failures stem from the reaction of the physiological environment with the metal, resulting in corrosion (dissolution), which either weakens the device mechanically or elicits an adverse tissue response necessitating device removal.
• Conjoint mechanical–environmental failures are produced by the combined effects of applied stress in a corrosive environment. Conjoint failure modes include fretting corrosion and corrosion fatigue.

Table 1. Potential metallurgical failure modes

Idiosyncratic failures refer to the selective rejection of an implant by certain patients, often associated with pain, hypersensitivity reactions, implant loosening, or sinus tract formation

Little is known about idiosyncratic failures; however, it is suspected that such failures originate from corrosion product-induced hypersensitization phenomena resulting in implant rejection or loosening. Alternatively, selective accelerated corrosion producing a direct toxic effect on neighboring cells has been postulated.

Poor patient compliance with the prescribed postoperative management program

Fatigue failure

Definitions

• Two main modes:

1. Fatigue
2. Creep

• Fatigue is failure at stresses well below the ultimate tensile strength of a material usually at low temperatures relative to the melting point in response to cyclical loading.
• Creep is the failure of a material usually at elevated temperature under a constant load in a time dependent manner.
• Other terms to be familiar with are ductile and brittle fracture.
• Ductile fracture patterns tend to show extensive plastic deformation and consequently absorb large amounts of energy before failure.
• Brittle fracture tends to deform very little therefore there is rapid crack propagation once initiated leading to unpredictable and catastrophic failure.

Fracture mechanics

• All materials contain flaws which mean fracture strengths are below predicted values.
• These flaws act as stress risers. The degree to which they amplify stress is dependent on their shape and orientation.
• This stress concentration is not limited to internal flaws but also to surface imperfections and screw holes.
• Knowing the likely stress a material will be subjected to and its fracture toughness (a material property quantifying the resistance to brittle fracture) it is possible to work out the maximal allowable flaw size.

Fatigue

• Accounts for the clear majority of metallic implant failures but can also occur in polymers and ceramics.
• Fatigue failure is similar to brittle fracture, in that it often occurs suddenly and shows very little signs of plastic deformation.
• Fatigue failure consists of three stages:

1. Crack initiation at a stress riser
2. Crack propagation with each loading cycle
3. Failure

Fatigue life

• This is often shown as S-N curves where applied stress and number of cycles before failure are plotted on the x and y axis.
• Various factors can be addressed to improve the fatigue life of an implant:
• Design – avoid sharp edges or transitions in radii.
• Reduce flaw size.
• Decrease the cyclical load – adequate fracture reduction.
• Improve surface finish by polishing.
• Impose compressive surface finish.
• Case harden.
• With cemented femoral implants Gruen described four modes of failure:

Mode 1: Pistoning behaviour

• 1a. A radiolucent line is seen between the stem and cement at the superolateral part of the stem. The stem is displaced distally, producing the radiolucent zone and a punched out fracture of the cement near the tip of the cement mass.
• 1b. A radiolucent zone can be seen about the entire cement mass, often with a halo or thin line of reactive sclerotic bone about the radiolucent zone.

Mode 2: Medial stem pivot

• This is caused by medial migration of the proximal portion of the stem. Lateral migration of the distal tip results from inadequate superomedial and inferolateral cement support. This may produce a fracture of the cement at the midstem and a fracture of the sclerotic bone lateral to the tip of the stem.

Mode 3: Calcar pivot

• This is caused by medial and lateral toggle of the distal end of the stem. The distal stem lacks support and a bone reaction develops. Adequate proximal support produces a windscreen wiper type of reaction at the distal stem, with sclerosis and thickening of the cortex medially and laterally at the level of the tip of the stem.

Mode 4: Cantilever bending

• This is caused by proximal loss of support of the stem while distally the stem is securely fixed. Radiolucent zones may develop proximally, medially and laterally to the stem, and may progress to stem failure.

Table 1. Modes of femoral stem failure

Figure 5. Greun Modes of failure

Aseptic loosening

• This is a multifactorial process which is directly related to wear-induced osteolysis.

Engh CA, Bobyn JD, Glassman AH.Porous-coated hip replacement. The factors governing bone ingrowth, stress shielding, and clinical results J Bone Joint Surg Br 1987; 69-B: 45-55.

Engh, Bobyn and Glassman studied the histological evidence for bony femoral stem ingrowth, its frequency, radiographic appearance and clinical features. They also assessed the importance of a press fit and of bone resorption and remodeling. Five year clinical results were examined

The authors describe in detail the radiographic signs of fixation and stability of a series of uncemented porous coated-cobalt chromium femoral stems.

The biological fixation of the stem was classified into bone ingrown fixation, stable fibrous fixation, or unstable.

The lack of reactive lines and the presence of spot welds around the porous surface were considered major signs of osseous integration. The absence of subsidence was considered a minor sign of stability.

Spot welds and proximal resorption were found in most hips that had stable fixation, and a lack of spot welds was a hallmark of unstable fixation. A pedestal was important if it was associated with an unstable stem tip.

The authors recommend implanting the stem with a tight press fit at the isthmus to achieve a predictable fixation. Poorly fitting stems were less likely to become fixed by bone ingrowth. Pain and a limp were less likely with a good press fit. Implants that filled the medullary canal poorly and/or had fibrous tissue ingrowth could be stable and remain so, but the clinical scores were significantly lower.

The femur appeared to respond to an extensively coated implant in 3 predictable modes that could be observed on plain radiographs. These modes were (1) bone growth occurs (2) bone ingrowth does not occur but the implant is stabilised by fibrous tissue ingrowth and (3) bone ingrowth does not occur and the stem becomes unstable.

1. Fixation by bone ingrowth

This was defined as an implant with no subsidence and minimal or no radio-opaque line formation around the stem. There is bony ingrowth at multiple points along the stem. There is no prosthetic subsidence and no radio-opaque lines around the porous coated portion of the stem. The pattern of bone remodeling involves new endosteal bone in contact with the stem, increase of cortical density at points of contact and proximal cortical atrophy in regions where the cortex is most distant from the stem

2. Stable fibrous ingrowth

This was defined as an implant with no progressive migration (slight early migration may have occurred) and extensive radio-opaque line formation around the stem. These lines surround the stem in parallel fashion and are separated from the stem by a radiolucent space up to 1mm in width. The femoral cortex shows no sign of local hypertrophy indicating that the surrounding shell of bone performs a uniform load carrying function.

3. Unstable implant

This was defined as one with definite evidence of either progressive subsidence or migration within the canal and is at least partly surrounded by divergent radio-opaque lines that are more widely separated from the stem at its extremities. Increased cortical density and thickening typically occurs beneath the collar and at the stem end, indicating regions of local loading and lack of uniform stress transfer

Stress shielding is a diffuse loss of bone density in the proximal aspect of the femur and is a favorable radiographic finding indicating that the femoral stem is well fixed distal to this area.

Engh, et al. reported a 12% rate of moderate to severe stress shielding (third or fourth de­gree) after cementless THA using an exten­sively porous-coated, cobalt-chromium alloy femoral stem.

Engh et al classified resorptive bone modelling secondary to stress shielding into 4 degrees of severity.

First degree

Only the most proximal medial edge of the cut femoral neck was rounded off slightly

Second degree

Rounding off of the proximal medial femoral neck was combined with loss of medial cortical density at level 1 in an AP film

Third degree

More extensive resorption of cortical bone typically involved both the medial and the anterior cortical regions at level 1 and the medial cortex at level 2

Fourth degree

Cortical resorption extends to below levels 1 and 2 into the diaphysis, with changes characteristically occurring in the medial and posterior cortices just above the level of the press fit where the cortex was most widely separated from the straight stem  

• The aetiology of stress shielding is multifactorial[7].
• The primary determinant of the adverse remodeling is a stiffness mismatch between implant and bone. The bending stiffness of an implant is proportional to the 4^th power of the radius. Therefore a small change in the stem size can have a marked impact on the stiffness of the stem.
• A fundamental principle of solid mechanics is when two materials are joined; the stiffer material or structure bears the majority of the load. Stress shielding seems to occur within the first 2 years following component insertion and after that time is not progressive
• Stem stiffness is the main factor affecting stress shielding. Factors affecting stem stiffness include stem diameter (~ radius 4 of stem), metallurgy (Co-Cr is stiffer than titanium alloy) and stem geometry (solid and round stems are stiffer)

Jasty M, Maloney WJ, Bragdon CR, O'connor DO, Haire T, Harris WH.The initiation of failure in cemented femoral components of hip arthroplastiesJ Bone Joint Surg Br 1991; 73: 551-8.

In an investigation into the initiation of failure in 16 retrieved femora after 2 weeks to 17 years of use, Jasty et al found cement debonding in almost all specimens but the cement-bone interface had remained intact in all cases.

The authors report on the mechanisms involved in initiating the loosening of cemented femoral components. In analyzed retrieved specimens it was found that cement fractures and disruption of the stem cement interface occur long before clinical or radiological failure of the total hip arthroplasty (THA) occurred. They suggested that local loosening of the stem-cement interface preceded cement failure.

The debonding starts at the most proximal and most distal ends of the prosthesis and progresses to involve the entire component over time.

Circumferential fractures in the cement near the cement-metal interface and radial fractures extending from the cement metal interface into the cement and to the cement-bone interface were common and associated with progressive loosening.

The most extensive cement fractures arose from the corners of those prostheses with sharp corners and in areas where the cement mantles were thin or incomplete. Cement fractures also occurred from voids in the cement. Rarely did they occur at the cement-bone interface.

The study improved the understanding of the mechanical aspects of femoral stem implant loosening and the initiation and development of cement damage. This is still an important clinical concern today.

Readers should cross reference the basic science material against the relevant joint specific clinical sections.

Exeter taper slip versus Charnley composite bean theory

Two fundamentally different design features for THA

1.Exeter taper slip

• Highly polished double tapered stem
• Collarless to allow controlled insertion
• Polished stem allows for subsidence within the cement mantle which maintains stability of the implant and protects against loosening
• The stem anticipates stem cement debonding and accommodates creep and stress relaxation in the cement mantle
• These stems characteristically creep up to 1mm over the first year
• Radial compressive forces are transmitted to the cement mantle and bone as hoop stresses
• To prevent stress on the cement distally during subsidence, an air filled centraliser is used to act as a cushion

2.Composite bean

• A Charnley stem is a matt finished, nonobloc, round back stem with a 22.225mm femoral head
• These stems often have a rough surface to improve bonding with cement
• Often have a collar
• Does not subside and forms bond between implant and cement
• The physiological load on the head of the prosthesis is transmitted through the metal stem to its tip and then to the cement and bone below it
• The prosthesis relies on obtaining a perfect bond between implant and cement
• If the stem subsides the bone cement interface has debonded and the implant is loose
• Require a thick cement mantle well fixed to bone

The effect of surface finish with the Exeter stem

• A polished surface finish is an essential feature of the Exeter stem design. However this is labour intensive and expensive to produce. When the stem was changed to a matt finish there was a higher than expected rate of failure (revision rate of 10% at 10 years)
• The matt finish led to stem bonding with cement and the taper slip was unable to settle in cement and convert the shear forces to compressive forces

Total knee arthroplasty biomechanics

• Often referred to as simple a hinge joint allowing flexion/extension in the sagittal plane.
• However, anteroposterior (AP) translation in the sagittal plane, internal-external (IE) in the transverse plane, and varus valgus motion in the frontal plane are also important for its overall function.
• The relative motions of the knee are described by three translations and three3 rotations – 6 degrees of freedom.
• Rolling predominates early in flexion (0–20°), whereas sliding (roll back) becomes dominant at flexion angles greater than 30°.

Four bar linkage

• This model is used to study the interaction of cruciates and the tibiofemoral joint.
• The model consists of two crossed rods representing the cruciates and the two connecting bars representing the tibia and femoral attachments of these ligaments.
• This mechanism has been used to describe the shape of both the tibial and femoral condyles, the path of the instantaneous centre of knee joint rotation, and the posterior migration of the tibiofemoral contact point that occurs with knee flexion.
• This model is, however, disputed as in practice the ligaments are not rigid and do not have a single isometric point at all positions of flexion. In reality there is variable tension in different bundles of fibres of the same ligament as different degrees of flexion.

Constraint in TKA

• Constraint is the effect of the elements of knee implant design that provides the stability needed to counteract forces about the knee after arthroplasty in the presence of a deficient soft-tissue envelope.

Non-constrained

• PCL is retained.

Semi-constrained

• In certain situations, such as patients with prior patellectomies, rheumatoid arthritis, or substantial preoperative deformities, a posterior-stabilised knee is preferred.

Constrained or hinged

• Recommended for patients with severe deformity or instability.
• Can be rotating hinge or fixed hinge.

Varus-valgus implant (posterior stabilised plus)

• Revision of condylar knee prosthesis that has increased constraint provided by an intimately fitting cam and spine mechanism that is broader and more elevated than a standardised posterior stabilised TKA.
• Its use is restricted to partial collateral ligament insufficiency or bone loss requiring augments to create an equal flexion/extension gap.
• Intramedullary stems are used to distribute forces over a larger area.

• Common causes of failure after TKA include infection, polyethylene wear/osteolysis, instability, aseptic (mechanical) loosening, extensor mechanism problems, aseptic necrosis of the patella, periprosthetic fracture, and arthrofibrosis.

PE wear

• Polyethylene wear is one of the main reason why TKA fail.
• Several factors influence PE wear. These can be grouped as:
• Material factors (method of sterilisation, shelf life, type of PE, manufacturing process, degree of oxidation, crosslinking).
• Surgical technique factors (soft tissue balance, implant alignment).
• Patient factors (activity, weight, gait pattern).
• This is directly related to contact stresses generated on the tibial insert during motion between the bearing surfaces.
• Wear can be minimised by reducing contact stresses.

PE thickness

• If the contact area is less than 8 mm thick, then the contact stress is greater than the yield strength of PE.
• It is important to realise that PE insert thickness is generally expressed as the thickness of the tibial base plate and PE together so that a 8 mm insert may be approximately 6 mm thick at its lowest point.

Articular geometry

• Conforming PE increases the contact surface area and reduces the contact load.
• A flat PE reduces contact area and increases point contact stresses.

Bartel DL, Bicknell VL, Wright TM The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg Am 1986; 68: 1041-51.

Bartel demonstrated the importance of conformity in prosthetic TKA design to increase contact area and to decrease contact stress. The greater the conformity, the greater the articular contact area with resulting reduced subsurface polyethylene contact stress per unit of area and therefore less polyethylene wear.

The authors used finite element techniques to analyze the role of polyethylene thickness on wear. These analyses indicated that bearing thicknesses less than 6 to 8 mm would result in increased maximum shear stresses occurring 1-2 mm below the surface. This depth corresponds to the location of cracking and delamination, also referred to as fatigue failure, seen in retrieved knee components. This is different from THA, where conformity can be achieved in all planes (a ball and socket articulation).

Bartel’s work underpins much later basic scientific and clinical studies of factors influencing polyethylene wear

Figure 6. Variation of the maximum (compressive) contact stress on the surface of the tibial component with varying thicknesses of the polyethylene layer. UHMWPE = ultra-high-molecular-weight polyethylene, C-UHMWPE = carbon-reinforced ultrahigh- molecular-weight polyethylene. 

• Knee instability
• Prosthetic mal-alignment

KPI is defined as the abnormal and excessive displacement of the articular elements that leads to clinical failure of the arthroplasty

It is one of the most common causes of failure following TKA

Instability may be early or late, and may involve global instability or instability in flexion, extension or recurvatum.

Early instability is typically caused by:

• Malalignment of the components,
• Failure of restoration of the mechanical axis of the limb,
• Improper balancing of the flexion–extension space,
• Rupture of the posterior cruciate ligament (PCL) or medial collateral ligament (MCL),
• Patellar tendon rupture or patella fracture.

Late instability following TKA is usually related to PE wear either alone or in combination with ligamentous instability

Implant longevity can be increased by more durable fixation, improvements in bearing materials, and lower polyethylene stresses Improvement in implant design and material would contribute to reduced incidence of failure after TKA

Efforts should be made to prevent failures related to surgical technique

Avoid implants associated with accelerated polyethylene wear and osteolysis,

Use of improved materials, such as cross-linking polyethylene,

Performance of precise surgical techniques to avert postoperative complications, such as instability and component malalignment or malposition (Computer navigation).

Constraint is the effect of the elements of knee implant design that provides the stability needed to counteract forces about the knee after arthroplasty in the presence of a deficient soft-tissue envelope.  

1.Non-constrained

PCL is retained

2.Semi-constrained

In certain situations, such as patients with prior patellectomies, rheumatoid arthritis, or substantial preoperative deformities, a posterior-stabilized knee is preferred. 

3.Constrained or hinged

Recommended for patients with severe deformity or instability

Can be rotating hinge or fixed hinge

Varus-valgus implant (Posterior stabilised plus)

Revision condylar knee prosthesis that has increased constraint provided by an intimately fitting cam and spine mechanism that is broader and more elevated than a standardized posterior stabilized TKA.

Its use is restricted to partial collateral ligament insufficiency or bone loss requiring augments to create an equal flexion /extension gap

Intramedullary stems are used to distribute forces over a larger area

Complications include prosthetic loosening, glenohumeral instability, periprosthetic fracture, rotator cuff tears, infection, neural injury, and deltoid muscle dysfunction.

• The humeral component can cause pain or stiffness if it becomes loose
• Excessive version of the humeral head can cause instability, limit motion or failure of tuberosity fixation
• Incorrect sizing of the femoral head, varus stem positioning, improper stem height can lead to an overstuffed shoulder that results in altered shoulder biomechanics leading to pain and stiffness
• Incorrect glenoid version or excessive glenoid wear can cause instability

Rationale

The basis of reverse shoulder arthroplasty is to create a fixed fulcrum at the glenoid against which the arm can be rotated by the deltoid, which cannot be addressed by unconstrained anatomic design prosthesis, leading to their failure in CTA

With the aid of the rotator cuff as dynamic stabilisers, the deltoid is able to elevate the shoulder against the glenoid as a fulcrum. In the absence of these stabilisers the humeral head migrates superiorly and impinges against the under surface of acromion, restricting the range of movement and causing pain 

Indications

• Non-functional rotator cuff muscles
• rotator cuff tear arthropathy with OA
• Massive, irreparable rotator cuff tear (without established OA)
• Rheumatoid arthritis, revision failed hemiarthroplasty & displaced proximal commuinuted humeral fractures elderly poor quality rotator cuff muscles & healing tuberosities

Biomechanics

Translates centre of rotation medially and distally

Biomechanically four main effects:

(1)   Lengthens lever arm deltoid so that it works more effectively absence of supraspinatous

(2)  Allows recruitment of more deltoid muscle fibers for elevation & abduction

(3)   Reduces torque on the glenoid component

(4)   Lowers the humerus compared to the glenoid which increases deltoid tension.

Reduces the shear force and increases the compression force across the the joint

Biomechanics of the ankle joint

• Motion at the ankle joint is a complex motion in three planes, mainly dorsiflexion and plantarflexion, but also eversion, inversion, internal rotation and external rotation
• The ankle transmits large axial (4-7 times body weight) loads and large shear forces (80% body weight). These forces are distributed over a small changing articular surface area. Therefore stresses produced are non- uniformly distributed in the joint.
• Ligaments and soft tissues are critical in maintaining the stability of the joint

Implant design

First generation

• Most designs consisted of two components: a polyethylene tibial component and a metal based talar component cemented into place.
• Either constrained or unconstrained although both had poor results
• Constrained hinge type implants had high failure rates mainly at the implant-bone interface due to large load transmission
• Unconstrained designs relied heavily on accurate positioning and soft tissue balance
• Failed due to instability in inversion/eversion and internal/external rotation

Second generation

• Semiconstrained design
• Examples include the LCS and STAR

Third generation

• Talar components shaped more anatomical

Fourth generation

• Characterized by a three-part, mobile-bearing, uncemented design

Failure TAR

• Malalignment is associated with pain, component edge loading, increased wear and higher failure rates.
• Good component alignment is considered instrumental for long term TAR success
• Design improvements have increased the success of TAR
• However revision rates of TAR are higher than those for hip and knee replacement 10-20% at 5 years
• Causes of failure include mal-alignment, periprosthetic fracture andaseptic loosening

Use of cement has been abandoned as the increased stresses applied to the bone–cement interface and the less well understood cement pressurization process lead to many early failures. Cementless fixations have bee more successful

Decreased bone resection to allow better bone quality for prosthesis fixation and improved surface biomaterials are key elements of newer design

• Linked or unlinked

Linked

• LInking of the humeral and ulnar components is done to avoid subluxation or dislocation episodes.
• Early linked implants were constrained hinges that only allowed flexion and extension.
• These implants were associated with a high failure rate secondary to the transmission of high stresses to the implant-cement-bone interface and other design flaws.
• Currently, most linked implants are semiconstrained: their linking mechanism behaves as a sloppy hinge, allowing some rotational and varus-valgus play.
• Semiconstrained implants are believed to transmit less stress to the implant interfaces, which associated with other design improvements have resulted in more reliable long-term fixation.
•  

Unlinked

• Unlinked implants dependent on the constrain gained from the articulating geometry of the implant
• Unlinked implants are attractive for patients with relatively well preserved bone stock and ligaments
• The most popular unlinked implants are the Souter-Strathclyde and the Kudo prostheses

Many surgeons favour linked implants, since they prevent instability and allow replacement for a wider spectrum of indications