Unicompartmental Knee Arthroplasty

Chapter Preview

CHAPTER SYNOPSIS

The results of unicondylar knee arthroplasty in the 1980s and 1990s were not very successful and the surgical procedure became unpopular in the United States. Repicci reintroduced the procedure with a minimally invasive surgical approach in the early 1990s. The indications for the surgery have been clearly defined and the surgical technique has been significantly changed. Unicondylar knee arthroplasty should not be performed as a total knee arthroplasty. Alignment and balancing are different because the surgery is performed on only one side of the knee. The knee must not be overcorrected, and flexion and extension balancing must be established without significant ligament releases. The slope of the tibial cut and the depth of the distal femoral resection can be used to adjust the balance. Partial knee arthroplasties are again becoming more popular, and unicondylar replacement is one of the significant choices.

IMPORTANT POINTS

1

Indications
- a
  
  History of pain in specific location
- b
  
  Physical examination showing site of pain, with correctable deformity
- c
  
  Radiographs showing varus less than 10 degrees, valgus less than 15 degrees, and flexion contracture less than 10 degrees.
2

Contraindications
- a
  
  Global pain
- b
  
  Global tenderness on physical examination
- c
  
  Radiographic deformity greater than limitations noted earlier
3

Surgical Technique
- a
  
  Balance knee with bone cuts.
- b
  
  Avoid overcorrection.
- c
  
  Avoid ligament releases or tightening.

CLINICAL/SURGICAL PEARLS

1

Patient selection is critical for a good result.
2

The ideal knee has the limited deformity that corrects to neutral with stress examination and has an intact anterior cruciate ligament.
3

The final result should have 2 mm of laxity in full extension and 90 degrees of flexion.

CLINICAL/SURGICAL PITFALLS

1

It is incorrect to choose the patient by radiographic criteria alone. The radiograph may be satisfactory but the patient may be overweight, or have global pain, or have an inflammatory arthritis that will preclude a good result.
2

The surgery must not overcorrect the knee alignment, and this can readily occur if the spacing is made too tight.

VIDEO AVAILABLE

Not available.

HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

Unicondylar knee arthroplasty (UKA) dates back to the early 1970s, when Marmor developed his replacement for the medial aspect of the knee. In the late 1970s, a mobile bearing prosthesis was developed, and UKA design work continued through the 1980s and 1990s. The results were not as good as those for total knee arthroplasty (TKA). In the first decade after replacement, the UKA was somewhat similar to TKA, but in the second decade, there were more failures and the interest in partial knee arthroplasty decreased. Insall and Aglietti’s report on UKA was especially discouraging with a high failure rate in the first 6 years. In the 1990s, Repicci and Eberle rekindled interest with their limited median parapatellar incision for a minimally invasive approach. Interest in UKA has continued with improved results but with a limited application. Most surgeons do not perform the operation frequently and, thus, it is difficult to develop a significant experience with the devices. However, with proper patient selection and improved surgical techniques, the newer results are similar to those of TKA with 15 years of follow-up.

INDICATIONS/CONTRAINDICATIONS

Initially, the indications for UKA were extremely limited. Insall and coworkers stated that only 7% of all knees qualified for the surgery. Osteotomy of the knee, debridement, cell grafting, allografting, osteochondral autograft transfer, and bicompartmental arthroplasty are other techniques that could be considered along with UKA. It is important to combine the patient’s history, physical examination, and radiographic evaluation to make a decision about the surgical procedure.

The history should be obtained from the patient with as much detail as possible. All of the conservative measures should be considered, including physical therapy, nonsteroidal anti-inflammatory drugs, intra-articular hyaluronic acid supplementation, and unloader-type bracing. The patient must be able to localize the site of the pain in a consistent manner. It is sometimes difficult to determine the extent of symptoms in the opposite tibiofemoral compartment or in the patellofemoral joint. If the patient describes a global pain in all zones of the knee, UKA is not appropriate. While it is somewhat questionable to lead the patient’s responses, in the population over the age of 80, UKA may represent the best limited approach for the patient despite the fact that the procedure will not relieve all of the patient’s symptoms. This discussion can involve much more time than the discussion for a TKA and the surgeon should be aware of this. However, the patients over the age of 80 represent some of the authors’ best surgical results. The expenditure of time is often very well worth the effort for the surgical result.

UKA has been described as the first arthroplasty for the very young and the last arthroplasty in the elderly. Patients who are 40 to 60 years of age form the younger population, and patients who are 80 years of age or older form the second group. Patients 60 to 80 years of age are more commonly considered for TKA but can also be considered for UKA if they have the appropriate indications. The younger group includes many of the same patients who are candidates for a high tibial osteotomy. Unicondylar replacement in this group permits an earlier return to full activity and is supported in the literature.

The results of tibial osteotomy are often used as the basis of comparison to UKA. The osteotomy studies report a 30% to 50% failure rate after 10 years. Conversion of the osteotomy to a TKA is not always a simple procedure. The authors agree that the quoted literature does not refer to opening wedge osteotomies, which return the tibia to a more anatomic configuration and may have a more successful long-term result that has yet to be published. Conversion of the UKA to a TKA is reported to have results similar to those of a primary TKA.

The elderly patients who are considered for UKA are often minimally acceptable for any surgical procedure. If the patient is willing to accept some pain in other zones of the knee, UKA is less physiologically challenging and is less invasive than TKA. The recovery is rapid with fewer complications and the longevity of the replacement is now similar to TKA.

Physical Examination

The physical examination begins with the patient’s height and weight. Either of these items alone may be misleading and the body mass index is probably more helpful. The author has arbitrarily chosen a weight limit of 250 pounds (114 kg). The literature has previously indicated a limit of 200 pounds (91 kg) with the concern of tibial component subsidence and/or excess polyethylene wear. The surgical technique should allow for this weight increase. Cortical contact of the tibial tray is critical, and dual peg fixation of the femoral component is helpful.

The knee examination should include the range of motion and degree of angular deformity. The surgical limitations for UKA are 10 degrees of flexion contracture, 10 degrees of varus deformity, and 15 degrees of valgus deformity. The varus and valgus deformity is easier to approach if the angulation is correctable to neutral on stress examination. Fixed deformity in the coronal plane increases the difficulty of the operative procedure.

The knee should have at least 105 degrees of flexion to permit enough range of motion in the operating room to insert the femoral component on the cut surface. There should be no medial or lateral thrust of the femur on the tibia through the stance phase of gait. A thrust implies increased laxity of the knee and correlates with translocation of the tibia beneath the femur on radiography leading to greater arthritic involvement of the opposite compartment. The medial and lateral tibiofemoral joint lines should be individually palpated along with the patellofemoral joint to determine the degree of tenderness. It is ideal to have the medial joint line most tender in the varus knee and the lateral joint line most tender in the valgus knee. The site of tenderness should correlate with the patient’s pain history. The ligaments of the knee should be intact but an absent anterior cruciate ligament is not an absolute contraindication to UKA if a fixed bearing device is used. However, dislocation can occur with a mobile bearing design in the anterior cruciate deficient knee.

Preoperative Planning

Plain radiographs form the mainstay of the imaging studies. The standing film is critical ( Fig. 5-1 ). A full-length study, including the hip, knee, and ankle, is desirable but not mandatory. Spot views of the hip and ankle are helpful to rule out referred pain. An appropriate patellar view, such as the Merchant view, will show the extent of the arthritic involvement and the alignment of the patellofemoral joint. The lateral film shows the patellofemoral joint and should be used to measure the slope of the tibia plateau ( Fig. 5-2 ). The normal plateau slope is 5 to 7 degrees with a range from 0 to 15. The preoperative slope of the tibia can be changed during the surgical procedure to accommodate a flexion contracture of the knee. A tunnel or flexed anteroposterior view of the knee is valuable to rule out involvement of the opposite femoral condyle with lesions in the area of the femoral notch.

Magnetic resonance imaging (MRI) studies are not mandatory but give further information concerning the status of the menisci, the ligaments, and any occult avascular necrosis (AVN) that may not be evident on the plain radiograph. The authors do not routinely request this study unless the patient’s complaints are suggestive of mechanical instability or an early AVN. If the patient undergoing a UKA has an undisplaced, degenerative tear of the meniscus in the opposite compartment, the authors will disregard this at the time of surgery. If the tear is displaced or if the patient also complains of instability, arthroscopic evaluation is recommended at the time of the arthroplasty. If there is an AVN present on the MRI, the surgical procedure must be delayed until the full extent of the lesion is clearly evident and the process is in remission.

Technetium-99 bone scanning can sometimes be helpful to determine if there is isolated unicompartmental disease or to compare the amount of uptake in one tibiofemoral compartment versus the other. Again, this technique is helpful as an adjunct but is not mandatory. MRI will often supplant bone scanning.

The alignment of the standing film can be measured using the anatomic axes. Ten degrees of varus or 15 degrees of valgus is acceptable. Deformity beyond these limits will require ligamentous releases. While releases have been very successful in TKA, it is very difficult to quantitate the releases in UKA and there is a strong tendency to overcorrect the knee and overload the opposite compartment after surgery. If the limits of 10 degrees of varus and 15 degrees of valgus are observed, the implant can be positioned with proper alignment and spacing to accommodate the prosthesis. Translocation of the tibia beneath the femur is a relative contraindication ( Fig. 5-3 ). In the younger population, translocation is not amenable to UKA because the impingement on the opposite femoral condyle will lead to pain and progressive deterioration with compromise of the result. In the elderly patient, with early translocation and predominant symptoms on one side of the joint, UKA may be possible. This is the group that will need to accept some residual pain in the other compartments of the knee as a compromise for the UKA procedure. The opposite compartment of the knee and the patellofemoral joint will almost always have some arthritic changes present at the time of the surgery. Mild to moderate involvement is acceptable for the UKA if the symptoms concur with the radiograph findings. The imaging studies should be combined with the history and physical examination to make an appropriate decision about the UKA.