If the ACL is ruptured and there is medial osteoarthritis, the erosions tend to extend to the back of the tibia so the disease is called posteromedial OA. In addition, there is usually cartilage loss on the posterior femur. The pathoanatomy is however different depending on whether the primary condition was anteromedial OA with secondary ACL rupture or primary ACL rupture with secondary OA.
With primary AMOA the disease begins anteriorly and centrally on the tibia with a varus deformity in extension. As the disease progresses posteriorly and the ACL fails, the varus deformity occurs also in flexion. The MCL shortens and the varus deformity becomes fixed. In addition the lateral subluxation of the tibia becomes fixed with increasing damage to the lateral side. Once these fixed deformities and lateral damage have occurred, the only solution is TKA.
In primary traumatic ACL rupture with secondary medial compartment arthritis, the cartilage defect and bony erosion tend to be central and posterior on the tibial plateau. This is likely to be due to recurrent episodes of giving way in which posterior femoral subluxation in the medial compartment can place a heavy load on the posterior horn of medial meniscus and posterior articular cartilage of the tibia, producing meniscal tears and arthritis. In some cases, the rest of the knee joint remains essentially intact with no shortening of the MCL. This is probably because, in extension, the intact distal femoral cartilage is in contact with the intact anterior tibial cartilage, so the varus deformity is corrected and the MCL is of normal length.
We, and others, treat such cases, in relatively young patients, by replacement of the ACL with a tendon graft and resurfacing of the medial compartment with OUKA. The operation is attractive to this group of patients who want to be active and hope to delay or avoid TKA. The ligament replacement and the resurfacing procedure can be done at the same time or in two stages. Pain is the usual presenting complaint. If this is the case, we address both pathologies offering a simultaneous UKA and ACL reconstruction. Occasionally, the patient presents with instability as the primary complaint and in these rare cases we offer ACL reconstruction first and only do UKA if pain subsequently becomes a problem.
Surgical technique for ACLR and OUKA
The set up is identical to the standard technique with the patient’s thigh supported on the leg holder. A standard incision is employed extending the distal portion below the tibial tubercule to facilitate hamstring graft harvest.
The OUKA is undertaken first. The spoons/clamp do not predictably help define the tibial resection height and more anterior bone is usually removed because the defect is posteromedial in location. Care is taken to identify the medial tibial spine and the vertical cut is made just off the apex of the spine. Femoral preparation and balancing are undertaken in the usual manner. The joint line may be raised slightly, as the foot of the femoral drill guide references off bone posteriorly, not normal articular cartilage. As a result a larger spigot than normal may be required to balance the knee. The anti-impingement device is then used to remove posterior osteophytes and adequate anterior bone. Care is taken to use as large a tibial component as possible and to ensure it is supported by cortical bone all around its periphery and the keel cut is made using the ‘keel cut’ saw. Attention is then turned to the ACL reconstruction.
We favour a hamstring graft for the ACL reconstruction. The tendons are harvested from the lower portion of the wound and sized in standard fashion. An appropriate sized femoral tunnel is positioned anatomically through the wound with the knee hyperflexed in a figure of four position. Care is taken to position the tibial tunnel to exit in the posterior half of the ACL footprint lateral to the vertical tibial cut. The tunnel is generally more vertical than usual. The Oxford components are then fixed either using cement or cementless and an appropriate sized bearing is snapped home. The ACL reconstruction is fixed to the femur using the surgeon’s fixation of choice. Finally the ACL is fixed to the tibia ensuring that the knee fully extends.
Results of combined ACLR & OUKA
We and others (Pandit et al, 2006; Weston-Simons et al, 2012), have achieved good results in this young, active and highly demanding subgroup of patients. In our series of 52 cases with mean follow up of 5 years (range 1 – 10), the mean age was 51 (range 36 – 57). The mean OKS was 41, 98% were pleased and the 10 year survival was 91%. In 11 cases, the ACL reconstruction had been done before the OUKA. The results were independent of how well the original ACL reconstruction was performed so we tend to accept the original ACL reconstruction providing it is functioning. A kinematic study comparing OUKA with intact and reconstructed ACL demonstrated that the kinematics following ACL reconstruction were nearly normal. The relationship between patella tendon angle and knee flexion was normal although the position of the bearing was slightly more posterior on the tibia with a reconstructed ACL than in OUKAs with an intact ACL (Weston-Simons et al, 2012 ).
OUKA with absent ACL
Although we have always advised surgeons not to do OUKA with a non-functional ACL, we have occasionally done this. These may be elderly unfit patients with correctable deformities but without an ACL. We would have justified this because the risks of UKA were substantially less than those of TKA. Also there are patients who are very keen to have a UKR rather than a TKR even though they have ACL damage. We have reviewed the outcome of these cases, with mean age of 65 years. At average follow up of 5 years, there was one failure as a patient developed lateral compartment OA at 3.4 years, needing a TKA (Boissonneault et al, 2013). A kinematic study has shown that ACLD patients function less well (slower step up) than those with an intact ACL (Pegg et al, 2015). Furthermore, the kinematics are less predictable with atypical bearing movements in mid flexion.
Despite the abnormality in kinematics the results in this small cohort are acceptable. However, the indications for OUKA without ACL are not defined. The indications are very narrow as it is only appropriate for a very small percentage of patients with OA and ACLD. If the indications were too broad, then a high failure rate would be expected as in previous studies (Goodfellow et al, 1988). Furthermore the surgery is more difficult with a deficient ACL because of the damage to the posterior tibia. We therefore continue to recommend that an OUKA should not be implanted with an absent or non-functional ACL (non-functional is demonstrated by ACL rupture when pulled with a tendon hook during surgery).
