Abstract
The reconstructive hip surgeon is commonly faced with complex cases where severe bone loss makes conventional revision techniques difficult or impossible. This problem is likely to increase in future, as there is a good correlation between the degree of bone loss seen and number of previous total hip operations. In such situations, one alternative is the use impaction allografting with cement. This has captured the attention of the orthopaedic community because of its potential for reconstituting femoral bone stock.
The first clinical reports of impaction allografting on the femoral side were in relation to revision with cementless stems. The use of morsellised bone with cement on the femoral side was first reported by the Exeter group. The great enthusiasm with which this technique has been received is related to its biological potential to increase bone stock. The rapid revascularisation, incorporation and remodelling of morsellised compacted cancellous allograft differs dramatically from structural allografting where bone ingrowth usually is limited to 2–3mm. Histological evidence for bony reconstitution has been presented from postmortem retrievals, and from biopsies at the time of trochanteric wire removal.
The size of the bone chips used as morsellised allograft is important. The graft behaves as a friable aggregate and its resistance to complex forces depends on grading, normal load and compaction. It is recommended that particles of 3–5mm in diameter make up the bulk of the graft. A bone slurry, such as that produced by blunted bone mills, or by the use of acetabular reamers or high speed burrs would not give satisfactory stability. A wide range of particle sizes is recommended in order to achieve the greatest stability. Future considerations will include the potential for either adding biomaterials to the allograft, or ultimately substituting it completely.
A satisfactory cement mantle is required to ensure the longevity of any cemented stem. The primary determinant of cement mantle thickness is the differential between the graft impactors and the final stem. All femoral impaction systems require careful design to achieve a cement mantle that is uninterrupted in its length and adequate in its thickness.
The technique of impaction allografting on the femoral side was first and most successfully reported using a highly polished stem with a double tapered geometry and no collar. It is thought to be ideal for this technique as it can subside within the cement mantle, thus generating hoop stresses on the cement which creeps, potentially maintaining physiological loads on the supporting bone. The extension of this technique to other stems has led to some controversy. Confounding factors such as surgical technique, the impaction system available, the type and size of allograft bone used, and the extent of the pre-operative bone loss, will undoubtedly continue to influence such comparisons. It appears that the exact stem configuration may not be as critical as its surface finish, the amount of graft impaction possible and the cement mantle produced.
Impaction allografting is the only technique currently available that reverses the loss of bone stock seen in a revision hip arthroplasty. Moreover, this technique does not sacrifice host tissue, and could facilitate further surgery. Impaction allografting, performed with great attention to detail using appropriate equipment, represents an exciting reconstructive solution for contained femoral defects. Its role in larger and combined defects remains open to scrutiny. Careful observation and cautious optimism are necessary as further refinements may well improve the predictability of the clinical results and expand the indications for this important addition to the armamentarium of the revision surgeon.
