eMedicine Specialties > Orthopedic Surgery > Knee

Unicompartmental Knee Arthroplasty

Author: Nanne P Kort, MD, Consulting Staff, Department of Orthopedic Surgery, Maasland Hospital Sittard, The Netherlands
Coauthor(s): Marcus Romanowski, MD, Consulting Surgeon, Department of Orthopedic Surgery, Joint Reconstruction Orthopedic Center; Jos van Raay, PhD, Associate Chair, Residency Director, Department of Orthopedic Surgery, Martini Hospital Groningen, Netherlands
Contributor Information and Disclosures

Updated: Jan 5, 2007

Introduction

Unicompartmental knee arthroplasty has had varying degrees of acceptance since its introduction approximately 30 years ago. Frequent and early failures were cited in initial studies. By the late 1990s, however, unicompartmental knee arthroplasty for the treatment of localized knee arthritis was becoming more common, and early outcome reports were favorable. Theoretical advantages of the procedure include preservation of uninvolved tissue and bone, reduced operative time, better range of motion, improved gait, and increased patient satisfaction. With appropriate patient selection, careful surgical technique, and proper implant design, unicompartmental knee arthroplasty can now be viewed as a procedure with reliable medium- to long-term success.

For excellent patient education resources, visit eMedicine's Foot, Ankle, Knee, and Hip Center and Arthritis Center. Also, see eMedicine's patient education article Knee Joint Replacement.

History of the Procedure

In the late 1950s and 1960s, MacIntosh and McKeever reported on the placement of a prosthetic disk into the diseased tibial plateau. The prosthesis was inserted into the affected side of the knee joint without cement. MacIntosh first used an acrylic prosthesis but later employed a metal one with a concave surface. The McKeever prosthesis had a small, T-shaped keel on its undersurface to improve fixation.

In the early 1970s, Gunston and Marmor independently introduced the cemented unicompartmental knee arthroplasty. Gunston employed a circular runner of stainless steel and a track of high-molecular-weight polyethylene, for use as either a bicompartmental or unicompartmental replacement. In 1988, Marmor published his 10- to 13-year follow-up of 97 cases. This report included 21 treatment failures attributed to development or progression of arthritis in the other compartments or to loosening, with subsidence of a thin (6-mm), all-polyethylene tibial component.

Laskin (1978) published a 2-year follow-up of patients using the Marmor modular knee replacement; he found that 35% of the knees had a fair or poor result, with high incidence of loosening of the implant and degeneration of the opposite tibiofemoral component. The revision rate was 22%. Insall (1980) published a series in which the conversion rate to a total knee arthroplasty was 28%.

Consequently, by the late 1980s, the popularity of unicompartmental knee arthroplasty had waned. However, after recognizing the need to avoid overcorrection of the mechanical axis, surgeons ultimately were able to reduce the risk that pain in the "normal" compartment would cause early failure. With the publication of several studies in the late 1980s and 1990s reporting 10-year survivorship rates exceeding 93%, enthusiasm for unicompartmental knee arthroplasty again increased, the survivorship rates being comparable to those for total knee arthroplasty. Although in 1994, the Swedish Joint Replacement Registry reported a high percentage of poor results for unicompartmental knee arthroplasty, these outcomes mainly reflected the fact that the surgery was performed on patients with chronic inflammatory arthritis.

Interest in unicompartmental knee arthroplasty was also stimulated by the introduction of the mobile-bearing form of the procedure. Goodfellow and O'Connor (1978), along with others, published excellent long-term survivorship rates associated with this technique. Their rationale for the procedure's success was clearly stated: a mobile bearing (also called a meniscal bearing) provides the unique combination of complete congruency of the articular surface (to minimize wear and creep) and total freedom of movement (to accommodate the preferred motion pattern of the retained natural compartment).

The past few years have again seen a heightened interest in unicompartmental arthroplasty, particularly because of the introduction of the minimally invasive parapatellar technique. This form of the procedure potentially can reduce morbidity, complications, and length of hospital stay. Many now view unicompartmental knee arthroplasty as preferable to high tibial osteotomy (HTO) in relatively young patients with medial compartment arthritis.

Problem

The concept behind unicompartmental knee arthroplasty is the replacement of only the damaged part of the knee and preservation of as much normal tissue and bone as possible to allow the restoration of normal kinematics.

Because unicompartmental knee arthroplasty is typically performed for medial osteoarthritis, this condition is the focus of this article. The surgical alternatives for treating the disorder—HTO, total knee arthroplasty, and unicompartmental arthroplasty—also are discussed.

Frequency

It is estimated that approximately 5-6% of arthritic knees are suitable for unicompartmental knee arthroplasty, depending on the examining surgeon's preference.

Pathophysiology

When anteromedial tibiofemoral compartment osteoarthritis develops, the posterior parts of the medial tibial and femoral articular surface remain relatively intact, so that, in flexion, the medial collateral ligament is under normal physiologic tension. Anteromedial osteoarthritis is also referred to as an extension gap disease. In the case of a rupture of the anterior cruciate ligament (ACL), the contact point in extension moves posteriorly, causing damage to the more posterior cartilage. The resulting loss of joint space in flexion allows the medial collateral ligament to remain shortened throughout the full range of movement of the knee, consequently causing a contracture of the medial collateral ligament. Instability and the change in kinematics cause progression of the arthritis into other knee compartments.

Lateral compartment disease can be thought of as a flexion gap disorder. It usually presents with sclerosis on the posterior tibial plateau, as well as subchondral collapse and erosions on the lateral femoral condyle.

Presentation

With medial compartment involvement, the patient usually presents with progressive signs and symptoms at the medial joint line. With primary lateral compartment involvement, the pain in the early stage is frequently minimal, and the patient often presents later in the course of the disease. Although the kinematics of the knee are quite complicated, the higher frequency of medial compartment arthritis and the fact that lateral compartment disease does not typically present at an early stage of its development are related to the valgus alignment of the knee's mechanical axis.

The diagnosis of unicompartmental osteoarthritis of the knee is made on the basis of patient history, physical examination, and workup findings.

A history of trauma should alert the surgeon to the possibility of a remote fracture or articular or ligamentous damage. A history of pain in multiple joints should draw attention to the possibility of inflammatory arthritis.

The physical examination is crucial for appropriate patient selection for unicompartmental knee arthroplasty (see Indications and Contraindications). The integrity of the ACL, the mechanical alignment and range of motion of the knee, and collateral stability must be assessed.

Indications

Careful patient selection is critical for unicompartmental knee arthroplasty if reliable results are to be achieved. The arthritis should be predominantly confined to a single compartment. (Medial compartment osteoarthritis is usually on the anteromedial aspect of the tibial plateau, and lateral compartment osteoarthritis is typically on the femoral side.) No significant degenerative changes in the other (medial, lateral, or patellofemoral) compartments should be present, and both cruciate ligaments should be intact. Absence of the anterior cruciate is a contraindication; the ACL makes the combined rolling and sliding at the meniscal femoral and meniscal tibial interfaces possible, which may yield near-normal joint kinematics and mechanics.

The operation is also indicated in patients with osteonecrosis of the femoral condyle. Not all of the unicompartmental replacements are suitable for the lateral side, because the ligaments of the lateral compartment are more elastic than those of the medial side.

Malalignment of the limb should be passively correctable to neutral and not beyond. This usually is possible in patients with a varus deformity less than 15° or a valgus deformity less than 20°.

The deformity of the knee should be only mild; therefore, a flexion contracture should be less than 15°. Unicompartmental knee arthroplasty with excision of osteophytes in the notch cannot correct moderately severe flexion contractures.

Ideally, the knee can be flexed to 110°. This is important for the preparation of the femoral condyle during the operation.

Relevant Anatomy

Despite a careful preoperative assessment, the ultimate determination of whether a patient is a candidate for unicompartmental knee arthroplasty may need to be made intraoperatively, keeping the indications and contraindications in mind. In some cases, it may be wise to obtain patient consent for unicompartmental or total knee replacement. Inspection of the supposedly normal compartment is essential; no significant degenerative changes should be present, and the meniscus should be normal. Peripheral osteophytes on the affected side must be identified and removed. The edges of the medial or lateral femoral condyle, the tibial plateau, and the intercondylar notch are the important surgical landmarks.

Contraindications

• Inflammatory arthropathy
• Previous HTO with overcorrection
• Sepsis
• Cruciate ligament lesion
• Medial or lateral subluxation (usually associated with a torn ACL)
• Tibial or femoral shaft deformity
• Flexion contracture greater than 15°
• Varus deformity greater than 15° (medial unicompartmental knee arthroplasty)
• Valgus deformity greater than 20° (lateral unicompartmental knee arthroplasty)
• Flexion less than 110°

Unicompartmental knee arthroplasty is controversial in the presence of patellofemoral joint arthritis, youth and high activity level, obesity, chondrocalcinosis, and crystalline arthropathy.

• Patellofemoral joint arthritis - Progression of osteoarthritis in the patellofemoral joint after unicompartmental knee arthroplasty is rare, according to some studies. In the Swedish Registry, no unicompartmental knee arthroplasties have required revision for patellofemoral problems. Murray (1998, 2005), Weale (1999), and their colleagues showed that residual postoperative pain was independent of the state of the patellofemoral joint, and no knee surgery was revised because of patellofemoral problems. Unicompartmental arthroplasty improves the mechanical axis and patellar tracking and allows more normal kinematics and rapid quadriceps rehabilitation. For these reasons, osteoarthritis of the patellofemoral joint may not be considered an absolute contraindication. However, other investigators and surgeons have reached the opposite conclusion; thus, many consider patellofemoral disease to be an absolute contraindication for unicompartmental knee replacement.
• Youth and high activity level - Because younger patients tend to be more active than older individuals, they would also seem likely to have a higher revision rate for unicompartmental arthroplasty than older individuals. However, although some studies have found such differences in revision rates, others have not. Minimally invasive techniques may reduce morbidity, complications, and length of hospital stay, which may favor the use of unicompartmental knee arthroplasty in younger patients.
• Obesity - Technical difficulties and increased risk of complications are associated with obesity. However, obesity is not considered a contraindication particularly for the unicompartmental knee arthroplasty mobile-bearing design. This is because a correlation has not been found between obesity and wear.
• Chondrocalcinosis - It may be necessary to differentiate between patients with a generalized chondrocalcinosis with synovitis and, effectively, an inflammatory condition in the knee (which is a contraindication for unicompartmental knee arthroplasty) and patients with calcification in the meniscus without generalized evidence of inflammation.
• Crystalline arthropathy - Many patients with osteoarthritis have calcium pyrophosphate crystal deposition in their articular surfaces and suffer from crystalline inflammatory arthropathy. Although Bumbry and Thornhill (2001) believe that crystalline arthropathy is a contraindication to unicompartmental replacement, Murray and colleagues (1998) disagree.

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