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.

An analysis of the cement mantle obtained with the Exeter impaction allografting system at one centre showed that it was either deficient or absent in almost 47% of Gruen zones. We therefore examined the mantle obtained using this system at another hospital and compared the results with those from the CPT and Harris Precoat Systems at other centres.

The surgical indications for the procedure and the patient details were broadly similar in all four hospitals. There was some variation in the frequency of use of cortical strut allografts, cerclage wires and wire mesh to supplement the impaction allograft. Analysis of the cement mantles showed that when uncertain Gruen zones were excluded, the incidence of zones with areas of absence or deficiency of the cement was 47% and 50%, respectively, for the two centres using the Exeter system, 21% for the CPT system and 18% for the Harris Precoat system.

We measured the difference in size between the proximal allograft impactors and the definitive prosthesis for each system. The Exeter system impactors are shorter than the definitive prosthesis and taper sharply so that the cavity created is inadequate, especially distally. The CPT proximal impactors are considerably longer than the definitive prosthesis and are designed to give a mantle of approximately 2 mm medially and laterally and 1.5 mm anteriorly and posteriorly. The Harris Precoat proximal impactors allow for a mantle with a circumference of 0.75 mm in the smaller sizes and 1 mm in the larger.

Many reports link the longevity of a cemented implant to the adequacy of the cement mantle. For this reason, femoral impaction systems require careful design to achieve a cement mantle which is uninterrupted in its length and adequate in its thickness. Our results suggest that some current systems require modification.

We have reviewed 25 cases of focal femoral osteolysis in radiographically stable, cemented femoral implants. In three hips retrieved at post-mortem from two patients, we have been able to make a detailed biomechanical and histological analysis. The interval between arthroplasty and the appearance of focal osteolysis on clinical radiographs ranged from 40 to 168 months, and in over 70% of the cases this did not appear until after five or more years. Few had significant pain and there was no relation to age, sex or original diagnosis. The most common site for osteolysis were Gruen zones 2 and 3 on the anteroposterior radiograph and zones 5 and 6 on the lateral radiograph. In 15 cases (60%), the area of osteolysis corresponded to either a defect in the cement mantle or an area of very thin cement. The rate of progression of these lesions was variable, but to date only one has progressed to gross loosening of the femoral component. The back-scatter scanning electron microscopic examination of serial sections and biomechanical testing of the post-mortem specimens demonstrated focal cement fracture around implants that were otherwise rigidly fixed. In eight cases from which tissue was available, histology showed a histiocytic reaction with evidence of particulate polymethylmethacrylate. We consider that this local fragmentation was the stimulus for local osteolysis in an otherwise stable cemented femoral component.