Introduction

Unicondylar knee arthroplasty (UKA) has progressed through two separate time phases since the original designs were developed in the early 1970s. The first phase was fraught with problems related to the prosthetic designs and patient selection.^1–4 The results were good to excellent for the first 10 years after the surgery in the hands of the designing surgeons. In the second decade the results did tend to taper off and were not as good as the reports of total knee arthroplasty (TKA).^5,6 It was difficult for the standard orthopaedic surgeon to reproduce the findings of the designers, and interest decreased in the late 1980s and early 1990s. Insall’s data showed that only 6% of knees satisfied the criteria for UKA, and he favored TKA as the procedure of choice.⁷

Repicci introduced the limited surgical approach (minimally invasive surgery, or MIS) for UKA in the early 1990s, and interest in the procedure increased by the year 2000.^8–13 Newer designs appeared, and the Oxford mobile-bearing UKA became very popular both in Europe and in the United States.^14,15 With this new wave of interest, surgeons looked to improve the clinical results and reviewed the patient selection criteria, the surgical approach, and instruments. If the incorrect patient is chosen, the result will be compromised despite excellent surgical technique and prosthetic design. This chapter outlines the factors involved in the choice process that should lead to a more satisfactory overall result.

History

It is important to understand the patient’s complaints and disability secondary to the arthritic knee. The underlying cause of the arthritis should become evident during the course of the interview. Inflammatory arthritis is not typically acceptable for UKA because the synovial reaction in the knee tends to involve all of the compartments of the knee in an equal fashion, and partial replacement will not adequately address the problem. Previous history of infection, obesity (with a body mass index > 33 or a weight > 225 pounds), and multiple ligament injury to the knee are relative contraindications. The patient should be able to identify the location of the pain on the joint line either medially or laterally. If the patient either cannot localize the pain or is confused about it, the procedure should not be considered. Patellofemoral symptoms are a relative contraindication, and if there are more symptoms with stair climbing than on level surfaces, UKA is probably not indicated. While the reports using a mobile-bearing UKA tend to ignore or deemphasize the importance of the patellofemoral joint, other authors have indicated that this area can lead to significant symptomatology and compromise of the result.

If the opposite knee has been replaced, the surgeon should evaluate the result with the patient. If the result of the previous surgery is excellent, the same procedure should certainly be considered for the other knee because the excellent result becomes the standard for comparison and will be difficult to equal and certainly more difficult to exceed. If the first result is equivocal, the choice for the second side is much easier. The pain should be localized and should be aggravated with activity and better with rest. If the pain is much worse with rest and at night during sleep, the diagnostic evaluation should be even more thorough to be sure that there is no other underlying condition, such as infection or inflammatory arthritis. If the patient has not had any previous replacements, the opposite side should also be evaluated at the same time with the same questions and discussion.

Laboratory Tests

There are some pertinent tests for UKA in order to better guarantee the clinical result. If the erythrocyte sedimentation rate and C-reactive protein are both elevated, the possibility of underlying infection should certainly be ruled out. Patients who are seropositive for the inflammatory arthritides (rheumatoid arthritis, lupus, and gout) should not have a UKA because of the prevalence of the synovitis in the entire joint space.

Physical Examination

The examination should include inspection of the gait and, then, the full evaluation of both lower extremities. There should be a component of antalgia to the gait, and any thrust of the femur on the tibia through the stance phase should especially be noted. As the deformity progresses either in the varus or the valgus knee, the collateral ligament on the compressed side of the joint shortens and the ligament on the tension side lengthens. This ultimately leads to shifting of the tibia beneath the femur with impingement of the lateral tibial spine against the lateral femoral condyle in the varus knee and impingement of the medial tibial spine against the medial femoral condyle in the valgus knee (Fig. 2–1). The shift of the tibia correlates with a lateral thrust of the femur on the tibia through the stance phase of gait in the varus knee and a medial thrust in the valgus knee (Fig. 2–2). This finding is a relative contraindication to UKA and should alert the examiner to correlate the physical finding with the standing anteroposterior radiograph.

Figure 2–1 The tibia has translocated beneath the femur with impingement of the lateral tibial spine into the lateral femoral condyle.

Figure 2–2 (A) In the varus knee, the femur will shift laterally on the tibia through the stance phase of gait as the deformity increases. (B) In the valgus knee, the femur will shift medially on the tibia through the stance phase of gait as the deformity increases.

The range of motion of the knee should be at least 5–105° of flexion. A flexion contracture of 5° can be partially corrected with the UKA; however, any greater degree of deformity will not be correctable and will lead to difficulty with the required flexion-extension balancing during the surgical procedure. The knee does not have to flex completely normally, but 105° will permit proper flexion exposure during the surgery and allow for functional motion afterward. UKA will not increase the preexisting motion.

The ligaments of the knee should all be intact for an ideal replacement. There will most certainly be some collateral ligament laxity as the deformity increases in either varus or valgus; however, there should be a distinct end point to the stress test for each collateral. The varus deformity should not exceed 5° and should correct to neutral on stress examination in the ideal case. The standard UKA does not include collateral ligament releases as in TKA (see Videos 2-1 and 2-2).

If the deformity is fixed and greater than 5°, the tibial cut will be deeper in order to accommodate the prosthetic thickness. This deeper cut can lead to increased loss of bone and metaphyseal fracture (Fig. 2–3). The valgus deformity can be as great as 10° but should correct passively to 5°.

Figure 2–3 (A) The postoperative anteroposterior radiograph shows the UKA well aligned but the tibial resection level is deep secondary to the fixed deformity of 10° that was too great for the procedure. (B) Fracture of the tibial metaphysis with some displacement distally but without angulation. (C) The fracture healed without repeat surgery.

Cruciate ligament deficiency is a relative contraindication. When the posterior cruciate ligament is torn, the drop-back of the tibia beneath the femur will lead to increased wear across the polyethylene surface and an earlier failure. If the anterior cruciate ligament (ACL) is torn and there is excess motion to either the anterior drawer test or the Lachman test, UKA is once again contraindicated. However, in most cases, the ACL may be torn at the time of the surgical procedure but there will be no significant laxity to the knee on physical examination. This often occurs because the knee has progressed with arthritis and the spurs and irregularities of the joint surface prohibit excess motion. When this is the case, the absent ACL is not a contraindication.

In the varus knee, the majority of the tenderness should be along the medial joint line. In the valgus knee it should be along the lateral side. These physical findings should correlate with the patient’s description of the pain. There may be a small effusion; however, if the effusion is large, the examiner should suspect more involved tricompartmental disease and be more hesitant to suggest a UKA. The extensor mechanism should be normal with no evidence of lateral patellar tracking (especially in the valgus knee). Slight patellofemoral crepitus is acceptable, but if there is marked crepitation with motion of the patella, the examiner should be more critical of the patellofemoral joint.

Imaging Studies

The primary imaging tool is the standing full-length radiograph. This allows the examiner to determine the mechanical axis of the limb and the associated joint space narrowing on the medial or lateral aspect of the joint. It is valuable to measure both the anatomic axis and the mechanical axis. There should be no greater than 5° of anatomic varus and 10° of valgus (Fig. 2–4). This should correlate with the physical examination findings. Translocation of the tibia beneath the femur on the standing view indicates that the disease is progressing with involvement of the opposite compartment (see Fig. 2–1). As such, it is a relative contraindication. The anteroposterior flexed view will show more detail of the posterior femoral condyles within the notch, and the posteroanterior flexed view will give more details about the loss of joint space on either the medial or lateral side. The lateral view will show the extent of patellofemoral disease, and if there is more than mild involvement, the patient should be examined and interviewed again to be sure that there are minimal symptoms attributable to this joint. In a similar fashion, there should be limited involvement of the opposite femorotibial joint on the radiograph, and this should also correlate with the history and physical examination. There is no doubt that there will always be a certain degree of arthritic disease in the entire knee; however, the primary involvement should be the medial or lateral femorotibial joint.

Figure 2–4 The ideal varus knee with medial narrowing and deformity less than 5°.

Magnetic resonance imaging (MRI) has become a very common tool for evaluation of the knee. Oftentimes, this study is requested before any radiographs are completed, and this is a mistake in the diagnostic chain. However, there are times when the MRI is valuable in combination with the appropriate radiographs. Sudden onset of distinct pain on the medial aspect of the knee often correlates with avascular necrosis, and it is important to make this diagnosis. If the event is recent, there will be hemorrhage into the medial femoral condyle or (less commonly) into the medial tibial metaphyseal area (Fig. 2–5). It is important to allow this early event to progress and mature with protected weight bearing before considering any UKA. If the hemorrhage is in the early phases, surgical intervention may lead to extensive loss of bone in the involved area and may require a TKA with complex augments to make up for the bone loss. After the avascular necrosis has matured, the remaining defect will be quite evident and it is usually surrounded by sclerotic bone that is much more amenable to UKA. On occasion, the patient may present with joint line pain medially and instability that may be secondary to pathology in the opposite lateral compartment. The author does not favor routine arthroscopy at the time of the UKA and does not favor routine MRI studies. However, MRI is a good tool to evaluate the lateral compartment and the lateral meniscus when there is a significant clinical suspicion. If the lateral meniscus is torn and the lateral compartment is also arthritic on the MRI, the surgeon should rethink the UKA and consider TKA.

Figure 2–5 (A) Avascular necrosis of the medial femoral condyle in the resolving phase. (B) Avascular necrosis of the medial tibial plateau in the resolved phase.

Computed tomography or arthrography of the knee are both infrequent studies but may be considered when a patient has a pacemaker and cannot undergo MRI evaluation. Technetium scans of the knee are sometimes valuable to pinpoint the area of primary arthritic involvement and also allow a visual comparison to the other areas of the knee (Fig. 2–6).

Figure 2–6 A technetium scan of a varus knee showing greater involvement of the patellofemoral joint than the medial joint, making UKA less desirable.

Conclusions

The results of UKA can be equally successful as TKA if the correct indications are followed.¹⁰ In a busy practice, UKA can represent 10–15% of the operative knee arthroplasty cases. It is extremely important to combine all three arms of the evaluation: history, physical examination, and imaging. If any one of these is questionable, it is best to abandon the UKA and consider TKA. However, if there are only relative contraindications in each of the three areas, the UKA can be performed with excellent results. The hesitant surgeon will often find reasons to abandon the UKA when the patient may very well be an excellent case for the surgery. The author has never abandoned the UKA during the operative procedure, and all decisions should be made well before the surgical procedure so that both the surgeon and the patient will be well prepared for the postoperative management and therapy.