Avoidance of extensor mechanism complications is best obtained by using components with an appropriately designed femoral trochlea and patellar component and where internally rotated components are avoided. Residual valgus deformity may also lead to patellar dislocation or more subtle subluxations, which may manifest as pain along the medial patellar retinaculum. Because rotational abnormalities are difficult to detect on plain x-ray, CT scans are a useful way to diagnose this problem. Peripatellar crepitation may cause symptoms and can be avoided by aggressive peripatellar synovectomy at the time of surgery. In its most severe form, the patellar clunk syndrome, most commonly seen in posterior stabilised knees, arthroscopic debridement of the offending fibrous nodule may be needed. Patellar fracture is best treated on the basis of residual extensor mechanism function. Maintenance of active extension following fracture (with no loss of component fixation) is a good sign that conservative therapy will yield better function than ORIF. Loss of extension indicates that surgery will be required. In this setting restoration of extensor continuity is more important than retention of the patellar component. Patellar tendon avulsion can be avoided by careful attention to the tendon during surgical exposure. Intraoperative repair alone is rarely successful and should be reinforced by semitendonosis, or fascia lata grafting. Late rupture may be treated by this type of grafting alone. But if the patella is necrotic or the failure results in a high riding patella, mechanism transplant is preferred. In this setting attention to fixing the graft in full extension is mandatory to prevent severe extensor lag, as the allograft will stretch out overtime.

Several centres have reported short-term (minimum 18–24 months) follow-up using cancellous impaction grafting with cement for femoral component revision, most often using stems of a similar geometry. The technique was described using the Exeter stem (Howmedica, Rutherford, NJ and Howmedica International, Staines, Middlesex, England), which is a double-tapered, highly polished, non-collared device; another implant commonly used for the procedure, the CPT stem (Collarless Polished Taper; Zimmer, Warsaw, IN), is similar in appearance. Advocates of the technique using these devices state that subsidence does not automatically lead to clinical loosening because the stem’s so-called “self-tightening geometry” allows re-stabilisation within the cement mantle as subsidence occurs. Cold flow of the cement mantle may help the stem to subside without becoming symptomatically loose. Subsidence of the wedge-shaped stem may also provide a beneficial compressive load to the bone graft.

However, other authors have raised concerns about the supposed benign nature of stem subsidence, and impaction allografting has been performed using stems that resist subsidence. Implants with a rough surface finish and polymethylmethacrylate precoating have been used.

When evaluating the published reports on impaction allografting, two important issues limit comparisons between clinical series. The most evident limitation is the inconsistent use of inclusion criteria in those papers. While many series have attempted to limit inclusion to femurs with more advanced stages of bone stock deficiency, others have specifically excluded some of those patients, as one of the originators of the procedure has expressed concerns about expanding the indications for impaction grafting to the most challenging femoral revisions. Some have been groups of consecutive patients undergoing femoral revisions, while still other studies do not define any criteria for inclusion at all. Another important limitation to consider when comparing clinical reports on this technique is the impressive number of variables that may impact on outcome in a femoral revision using impaction allografting. Two series using similar implants and similar inclusion criteria may still differ with respect to cement (technique, type, viscosity), allograft (source, consistency, pretreatment with radiation or freeze-drying), surgical approach, and aftercare, to name but a few potentially important factors. The effects of most of these variables on results in this especially complex technique have yet to be described.