The epicondylar attachments of the collaterals, which were preserved during exposure, are critical in the securing of the femoral APC. As in the tibia, the femoral canals are reamed to securely fit a stemmed implant with proximal fixation into the host bone of two cortical diameters or a minimum of 5
cm. On the back table, the femoral APC is prepared with the revision cutting guides to make the appropriate bone resections (see ). The epicondylar attachments of the collaterals are secured to the allograft through transosseous drill-hole tunnels where the collateral ligaments would be in a native distal femur. Sutures are passed through these tunnels and left long to attach the host collaterals once the APC is implanted.
Intraoperative pictures of allograft-prosthesis composite (APC), AP view (a) and lateral view (b).
The trial femoral components with their securely fitted stems are implanted into the host diaphysis. The flexion and extension gaps are checked and adjusted as needed. If the extension gap is tight, distal femoral resection of the allograft is performed, and, if the flexion gap is tight, the components are translated anteriorly or downsized. If both flexion and extension gaps are tight, we recommend adjusting cuts on the femoral side and downsizing rather than taking any more of the native proximal tibia. This will also ensure that overstuffing of the knee does not occur and makes wound closure less difficult.
When it is time to implant the stems of either the femoral or tibial side, a critical principle is to avoid cementing of the stems to the host bone. Conversely the allograft side of the stem and implant-allograft interface must be cemented to provide stability to construct. Meticulous cement technique needs to be utilized to ensure the allograft-implant interface has the requisite stability to allow early motion and rehabilitation (see ). Thus, a copiously irrigated allograft, which is carefully dried, is requisite prior to cementing. Use of low-dose antibiotic containing cement is acceptable; however, we do not add additional antibiotic to the cement as it weakens it and may potentially result in a poor cement mantle. The cement is allowed to harden the APC, and, once this is done, it is implanted into the host canal through a press fit. Rotational position should be aligned to the previous cautery or marker line as it is impacted. No cement should be present between the allograft-host bone junction as it would potentially interfere with graft incorporation. We emphasize avoidance of cementing stems to the host bone as it can make future revisions extremely difficult.
Radiograph showing a supracondylar periprosthetic fracture with major bone loss ((a) and (b)). An AP radiograph showing a revision with femoral allograft-implant composite (c).
Once components are implanted, the collaterals are attached using the previously placed heavy suture into the allograft epicondyles. Roughening up the allograft epicondyles and suturing the host epicondylar bony wafers may assist in incorporating the ligaments to the APC. Collaterals are tightened maximally in 90 degrees of knee flexion. Supplemental cerclage wiring of the remaining epicondyle host bone can be done to reinforce the sutures.
At this point, we place morselized autograft at the host-allograft junction and attempt to suture a periosteal or synovial flap around the autograft to secure it. Additional fixation may be required if the step or oblique cuts do not impart adequate stability. We suggest using additional screws rather than a cortical strut, as the strut increases bulk to the construct and may compromise the soft tissues. Similarly, our preference is to avoid plate fixation to the allograft as multiple drill holes weaken the graft and make it susceptible to fracture or accelerated vascularization and resorption. This can be a catastrophic complication.
Overall stability of the knee is rechecked with the implants in situ. It should be anticipated early if a highly constrained implant is required based on physical exam and imaging. It is subtler in determining whether a posterior stabilized polyethylene insert or varus-valgus high-post constrained liner is required. We prefer to use the least constraint possible to avoid transfer of stress to the APC interface.
In general terms, we avoid the highly constrained implants such as a rotating hinge implant, as the force transfer to the APC junction is significant and may lead to early failure.