Introduction

When conservative measures offer no permanent solution for pain caused by medial or lateral knee arthritis, surgeons should consider a corrective tibial head osteotomy, unicompartmental knee prosthesis, or total knee prosthesis. In patients with medial or lateral knee arthritis, a unicompartmental knee prosthesis seems a logical choice because the opposite joint compartment is not damaged.

In the past decade, the unicompartmental knee arthroplasty (UKA) has increased in frequency worldwide due to improved surgical technique, instrumentation, and component design.^1-4

History

The first unicompartmental knee prostheses were implanted in the 1950s and 1960s by MacIntosh⁵ and McKeever,⁶ respectively. The medial tibial plateau was replaced by an acrylic or steel component. In the early 1970s, Marmor⁷ introduced the first cemented metal (femoral component) on polyethylene (tibial component) unicompartmental knee prosthesis. In 1988, Marmor^8,9 published disappointing long-term results after 10- to 13-years' follow-up. In approximately 20% of patients, polyethylene wear or asymptomatic arthritis of the contralateral compartment was reported. The Swedish Joint Replacement Registry also published unsatisfactory results, mainly because the unicompartmental knee prosthesis was also implanted for chronic inflammatory conditions.¹⁰ Other investigators also indicate revision surgeries in 20% to 28% of patients.^11,12 For these reasons, unicompartmental knee prostheses became less popular in the 1980s.

Recently, the reports of UKA have been encouraging.^13-16 The better results are attributed to improved prosthetic design and surgical technique, and precise patient selection.

Prostheses

New prosthetic designs have been introduced, and the reduction of wear rates is an issue.^17,18

In a fixed-bearing UKA with a flat tibial component, the articular surface is not congruent and point loading occurs (Slide 1 and Slide 2).¹⁹ A mobile-bearing UKA is designed to allow large areas of contact between the femoral and tibial components, reducing the wear rate (Slide 3 and Slide 4).^20,21 The linear penetrative wear rate of a fixed-bearing UKA is 0.15 mm/year¹⁷ compared to 0.036 mm/year for a mobile-bearing UKA.¹⁸ In a retrospective study comparing fixed- and mobile-bearing tibias in 2002 by Emerson and colleagues,²² no difference in clinical outcome was identified. A recent prospective, randomized study demonstrated no statistically significant clinical advantage between the two tibial bearings.²³ In one study, the short-term (2-year follow-up) complication rate and clinical outcome results were poor with the mobile-bearing tibia.¹⁹

SLIDE 1		SLIDE 2		SLIDE 3		SLIDE 4

The ratio of UKA that involves the medial vs the lateral compartment is approximately 10:1.²⁴ The medial and lateral compartment replacements cannot be compared because of the difference in the biomechanical characteristics of each.²⁵ When the mobile-bearing tibia is used in the lateral compartment of the knee, approximately 10% of the bearings dislocate.^26,27 The mobile unicompartmental replacements are not suitable for the lateral side because of the kinematics, and ligaments of the lateral compartment are more elastic than those of the medial side, which enhances the dislocation tendency.

The Minimally Invasive Technique

The advent of minimally invasive surgery has allowed UKA with less morbidity. The introduction of the minimally invasive technique contributed to the renewed enthusiasm for the unicompartmental procedure. Repicci²⁸ showed that it is possible to perform the procedure through a 3-inch incision.^29-33 The surgery is performed through a short incision from the medial or lateral pole of the patella to the tibial tuberosity. Minimal damage to the extensor mechanism occurs, the patella is not dislocated, and the suprapatellar synovial pouch remains intact. As a result, patients recover faster. Knee flexion, straight leg raising, and independent stair climbing are achieved three times faster than after total knee replacement and twice as fast as after open UKA.³⁴ A minimally invasive procedure is a reliable and effective procedure.³² Midterm comparison between the conventional approach and the minimally invasive approach shows no difference between the knee scores and flexion/extension.²⁸ With appropriate pain control, the procedure can be safely performed as an outpatient procedure with substantial cost savings, and patients report less pain postoperatively compared to preoperatively.³⁵

Indications

In 1989, Kozinn and Scott³⁶ published the most widely accepted indications for UKA. Their criteria included age >60 years, weight <82 kg, low activity level, and minimal pain at rest. The minimum arc of motion is 90° with no more than 5° of flexion contracture. The angular deformity should be <10° of varus and <15° of valgus and minimal or no involvement of the adjacent tibiofemoral and patellofemoral compartments.

The presence of the anterior cruciate ligament may play a critical role as it makes the combined rolling and sliding at the meniscal femoral and meniscal tibial interface possible, which may yield near normal joint kinematics and mechanics.³⁷

With the encouraging results and the newer techniques and designs, indications for the procedure are broadening.

Studies suggest that any degree of obesity has a negative effect on the outcome of total knee arthroplasty. To date, there are few studies concerning the outcome of UKA in obese patients. Obesity can cause technical difficulties and a increased risk of complications.^14,38,39

Recent studies suggest that the results of UKA in a young patient group compare well with the results of total knee arthroplasty in the similar age groups.^14,40,41 Nonoperative management must be exhausted before surgery is contemplated. A high tibial osteotomy may be applicable in a young patient.

Patellofemoral osteoarthritis may be a contraindication when patellofemoral symptoms are present preoperatively.^42-44 Patellofemoral osteoarthritis does not influence the clinical outcome, does not progress after a mobile-bearing UKA, and is not a reported reason for revision.⁴⁵

Patients with inflammatory diseases such as rheumatoid arthritis are not candidates for mobile-bearing UKA because of synovial involvement of the opposite compartment.^46,47 Knee arthroplasty in rheumatoid arthritis is associated with progression of rheumatoid arthritis and loosening of the components.⁴⁸

Advantages Over Upper Tibial Osteotomy

Unicompartmental knee arthroplasty and upper tibial osteotomy have been used to treat unicompartmental arthritis in similar patients. For some surgeons, corrective tibial head osteotomy is losing its position as the gold standard.^49-52

A comparative study by Broughton and colleagues⁵¹ into the results of a corrective tibial head osteotomy against those of the unicompartmental knee prosthesis show good results, 43% and 76% respectively. Late degeneration in the lateral compartment was not seen after placement of a unicompartmental knee prosthesis. Weale and Newman⁵⁰ report better function and longer survival of the unicompartmental knee prosthesis compared to a tibial head osteotomy. More studies show a quicker full recovery, easier rehabilitation, fewer perioperative complications, and better long-term results after the unicompartmental knee prosthesis compared to a tibial head osteotomy. Conversion of a tibial head osteotomy to a total knee replacement is associated with a higher complication rate, and the results are generally not similar to those of primary total knee replacement.⁵³

Advantages Over Total Knee Replacement

Long-term follow-up results of UKA demonstrate predictably good results comparable with those of total knee replacement.^3,32,54

Unicompartmental knee arthroplasty has multiple advantages over total knee arthroplasty. The surgical approach and surgical dissection can be considerably less extensive for UKA,^29,55 with a decreased transfusion requirement.⁵⁶ Lower implant costs,⁵⁷ greater range of motion,^56,58 and a faster recovery make a shorter hospital stay possible.³⁴ Additionally, performing a conversion of a UKA to a total knee replacement is not as difficult as a revision total knee replacement.^59,60

Disadvantages of Unicompartmental Knee Arthroplasty

Approximately 6% of patients with arthritis of the knee are appropriate candidates for UKA.⁶¹ With the minimally invasive technique, the visual field is restricted, which makes UKA a demanding procedure.^53,62 The nature of the implant, careful patient selection, the experience of the surgeon, instrumentation, and the surgical technique are important.^53,63 For UKA, the long-term results are related to the number performed by the unit; the relatively small number of procedures for a surgeon may lead to compromised results.⁵³ This implant should only be used by surgeons who trained in its use with an understanding of how to avoid common errors.^53,64 To minimize errors and improve the long-term follow-up, knowledge of the pitfalls and the tips and tricks is obligatory.

Discussion

Unicompartmental knee arthroplasty offers an alternative to tibial head osteotomy and total knee replacement in cases of isolated unicompartmental osteoarthritis. With preservation of the joint, including the anterior cruciate ligament, normal knee kinematics are retained with better functional results. Excellent long-term results are comparable to those of total knee arthroplasty. Revision of failed UKA to total knee replacement can be accomplished successfully but may be associated with technical difficulties depending on the technique of the original surgery. A minimally invasive technique offers advantages with regard to faster short-term recovery over the traditional approach.

With appropriate patient selection, prosthetic design, and surgical technique, a good outcome can be achieved. Unicompartmental knee arthroplasty is a technically demanding surgery, and the surgeon should be well trained in its indications and technique.

Improved results of UKA have led to the broadening of its indications. Studies are being performed to determine whether good long-term results can be achieved in younger, active patients and obese patients. Newer techniques of implantation, better instrumentation, and updated prosthetic designs may extend the indications and improve the implant survivorship.

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