Bassam A. Masri, M.D., FRCSC
Associate Professor and Head
Division of Lower Limb Reconstruction and Oncology
Department of Orthopaedics
University of British Columbia
Vancouver, BC

How often have you entered a knee joint at the time of a knee replacement to find that the arthritic changes are confined to the medial compartment, with a normal lateral compartment, near normal patellofemoral compartment and a normal anterior cruciate ligament? This occurs perhaps once every four to eight knee replacements.

As a surgeon whose most frequent operation is a knee replacement, I was faced with that situation quite often. A knee replacement is a good operation with good long term results; however, it is not a normal knee and does not feel like a normal knee to the patient. Invariably, the range of motion is less than that of a normal knee because most knee replacements are not designed to give a full range of motion. Furthermore, most patients cannot kneel on their knees after a knee replacement. While this may not be that important for most patients, others wish to continue to be able to kneel, for a variety of social and religious reasons.

Figure 1: The removed tibial plateau cut during a UKR. Note the intact articular cartilage on the posterior aspect of the cut.

Let us consider patients with isolated medial compartment disease. These patients are often younger, more active, are often working and demand more of their knee. Their preoperative knee function is usually good, with the exception of the pain and deformity, and they generally come to the surgeon expecting pain relief and return to function. Some patients require a rapid return to work. Across the country, these patients are usually offered one of four options. One option is conservative treatment, which has usually been exhausted by the time the patient finally sees the surgeon after a wait of a few months to over one year in our overburdened health care system. This option is usually unacceptable to most patients, who come to the surgeon seeking a surgical solution. The surgical options include a proximal tibial osteotomy, with its inherent lack of predictability, and almost mandatory requirement for revision to a total knee arthroplasty at 6-10 years at best. Another option is a total knee arthroplasty, with its inherent limitations as outlined above. The final option is a minimally invasive unicompartmental knee replacement.

The concept of isolated medial compartment osteoarthritis of the knee is not new. White el al¹ described the features of anteromedial osteoarthritis of the knee. In this condition, the arthritic changes are restricted primarily to the medial compartment, with the tibial damage primarily restricted to the anterior aspect of the tibial plateau. The posterior portion of the tibial plateau is often preserved (Figure 1). Radiographically, a central erosion of the tibia is seen, with an intact posterior tibial plateau on the lateral view (Figure 2). This finding is important because it suggests that the tibia cannot sublux anteriorly on the femur, and that the anterior cruciate ligament is therefore intact. If the posterior portion of the tibial plateau is not intact, the anterior cruciate ligament is incompetent, and these patients should not be offered a unicompartmental knee replacement. The other feature of this condition is the presence of full thickness articular cartilage in the lateral compartment. This can be evaluated with a valgus stress radiograph with the knee in 20o of flexion. If the knee alignment corrects from its varus position, and the lateral compartment appears to have full articular cartilage thickness, then a unicompartmental knee replacement should be considered. If not, then the operation should not be offered. The indications for a unicompartmental knee replacement are summarized below:

1. significant pain related to medial compartment osteoarthritis;
   
2. the pain is mostly medial, with no significant anterior or lateral pain;
   
3. varus deformity not larger than 15°;
   
4. fixed flexion deformity not larger than 15°;
   
5. flexion greater than 115°;
   
6. correctable varus deformity;
   
7. intact lateral compartment;
   
8. intact anterior and posterior cruciate ligaments;
   
9. in the absence of anterior knee pain, the status of the patellofemoral joint is irrelevant, unless there is severe degeneration.
   

The advantages of a unicompartmental knee replacement (UKR) (Figure 3) are that the operation can be done through a smaller incision. In my hands, this is a 3" incision (Figure 4) as opposed to a 5" incision for a total knee replacement (TKR) in a non-obese patient. The actual length of the skin incision, however, is of little relevance. The main advantage is that the arthrotomy for a UKR is only through the capsule, and the quadriceps tendon is not interfered with. In a TKR, the approach requires eversion of the patella and in my hands, a medial parapatellar arthrotomy that goes through the quadriceps tendon, while others recommend the use of a subvastus or a mid-vastus approach. Another advantage is the rapidity of early rehabilitation after a UKR. Typically, patients have no pain immediately after surgery. They can straight-leg raise and flex their knees immediately. This is often difficult after a TKR. Blood loss is typically much less after a UKR. The knee is not dislocated and therefore there is less kinking of the popliteal vessels, and at least theoretically, there is less risk of thromboembolic disease. The length of hospital stay after a UKR is typically much shorter. In my practice, the patient is discharged the morning after surgery. The only reason for overnight stay is the administration of postoperative prophylactic antibiotics. The patients typically walk into the office at their 1-2 weeks postoperative visit with only one cane at most, and quite often without any walking aids. The motivated patient can return to sedentary work at one week postoperatively, and therefore the cost in terms of reduced productivity is much less than that of a knee replacement. The other advantage is that this operation lends itself very well to being performed at a non-hospital surgical facility in the private sector. For third party payers, this minimizes the wait for surgery, so that the patient can return to gainful employment more quickly, lessening the financial burden on the third party payer and on society as a whole. Finally, the final functional outcome is often better than a TKR in terms of range of motion and the ability to kneel.

Figure 3: Anteroposterior (A) and lateral (B) radiographs showing a UKR in situ.

But, even though the pain is less, the hospital length of stay is less, and the early functional result is better, does this operation work in the long term? The answer is an unqualified yes. Savard et al² reported a 95% (95% CI 90.8%-99.3%) reported survivorship at ten years, where the end-point was revision for any reason, using the Oxford knee (Biomet, Warsaw, IN). Similarly, Berger et al³ reported a 98% survivorship at 10 years using the Miller-Galante UKR (Zimmer, Warsaw, IN). These long term results are not different from the results of total knee arthroplasty.

In conclusion, for the well-selected patient who meets all of the requisite indications, minimally invasive unicompartmental knee replacement is not only a viable solution, but perhaps the preferred surgical solution for anteromedial osteoarthritis of the knee. At the present time, there are substantial reservations in the literature regarding the use of UKR in lateral compartment osteoarthritis, and the arguments put forth in this article should not be used as justification for lateral compartment UKR.

References

1. White S., Ludkowski P.F., Goodfellow J. J Bone Joint Surg 73[B]:582-586, 1991.
   
2. Savard U.C., Price A.J.: Oxford medial unicompartmental knee arthroplasty. A survival analysis of an independent series. J Bone Joint Surg 83[B]:191-194, 2001.
   
3. Berger R.A., Nedeff D.D., Barden R.M., Sheinkop M.M., Jacobs J.J., Rosenberg A.G., Galante J.O.: Unicompartmental knee arthroplasty. Clinical experience at 6- to 10-year follow-up. Clin Orthop 367:50-60, 1999.