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