Impaction allografting

The principle of impaction grafting is to restore cavitary meta‐diaphyseal bone loss with morselized allograft to support a cemented stem (Figures 33.1 and 33.2). Poor‐quality cement mantle (≤2 mm) appears to influence migration of the femoral stem.⁶ The greatest migration, typically into varus and retroversion, occurs in the first three months and is attributed to further graft compaction and a poor cement mantle,⁶,⁸ which is often due to poor instrumentation.²,³,⁶ Furthermore, cement mantle defects may cause progressive migration during creeping substitution.² Masterson et al. in their study of multiple stem techniques documented concerns with the quality of the cement mantle due to inadequate instrumentation.⁹ The Exeter system lead to absence of cement in 50% of Gruen zones, which was attributed to the trial impactors being shorter and more sharply tapered, which created an inadequate cavity. The CPT and Harris Precoat systems led to an absence of 21 and 18%, respectively. In a later study, design improvements in a commercially available system utilized with Exeter stems allowed an improvement of 23.4% of Gruen zones absent of cement.¹⁰

Vigorous pressurization of the cement can lead to its contact with the endosteal bone, which improves overall stability of the construct, but also potentially impacts revascularization and remodeling.¹¹ Adequate size of the graft particles to achieve interfragmentary stability¹² and quality of graft impaction are also important to prevent subsidence and poor outcome.⁴,⁶ Ultimately, a combination of construct stability and graft incorporation and remodeling must be obtained. When an extended trochanteric osteotomy (ETO) is performed, the ETO is closed first prior to impaction. With older designs, a cortical only strut allograft was used to reinforce the osteotomy. However, with the advent of long stems for impaction allografting this is no longer necessary.⁷

Tight allograft packing is critical, which may lead to intraoperative fractures. Adequate exposure, prophylactic cerclage wire or cable fixation, and avoidance of bending stresses and torque within the femur at the time of impaction are important.⁷ In addition, bone defects can be reinforced with specialized wire mesh or with cortical only allograft struts (Figure 33.1). A variety of instrument systems have been introduced to help standardize the impaction technique and assess stability.¹³ Longer stems with or without additional fixation may be required for added stability and should bypass weakened regions by two cortical diameters.⁷

Proximal femoral allografting

The surgical technique for proximal segmental femoral allografts is demanding and is typically reserved for selected cases of Paprosky type IIIB and IV defects. Possible surgical approaches that may be used include either a trochanteric slide,¹⁴ which may reduce trochanteric migration, or a trochanteric splitting osteotomy.¹⁵ The length of the allograft required depends on the degree of bone loss, stability, and leg length assessment which is determined from preoperative templating and intraoperative assessment. Soft tissue should be preserved as much as possible and every effort should be made to preserve the greater trochanter.

Allograft bone is biologically inactive and cannot grow into cementless implants; therefore, the prosthesis should be cemented into the allograft.⁵,¹⁶ The distal aspect of the stem can be cemented or uncemented; however, care should be taken to ensure that no cement remains at the host bone/allograft junction. Additionally, autogenous reamings from the acetabulum, or from the resected proximal femoral bone, can be utilized along the allograft–host junction to facilitate union. The canal of the allograft should not be over‐reamed so as to preserve its strength. If the host canal is larger, as is often the case, a telescoping method can be used,¹⁷ during which the smaller allograft segment is telescoped into the large and vacuous distal host femur. The addition of a step cut or oblique cut with cable reinforcement, plate fixation, strut allografts, and distal press fit stems aim to improve the stability of the construct to promote union.¹⁶ However, if plate fixation is chosen, the portion lying on the allograft should not be fixed with screws, but instead by cables, so as to avoid fatigue fractures. Proximal femur remnants, with their intact soft tissue attachments can then be wrapped around the construct to provide improved soft tissue attachments (Figure 33.3), and to provide a vascularized graft around the allograft–host junction.