Unicompartmental Knee Arthroplasty
Nanne P. Kort, MD, PhD
Unicompartmental arthritis of the knee is defined as a condition characterised by degenerative articular cartilage in the medial or lateral aspects of the tibiofemoral joint, which may be associated with meniscal disruption, ligamentous instability and malalignment.1, 2It is the most common degenerative joint disease. The prevalence of this symptomatic condition in adults over age 65 is 10%. The prevalence of radiological disease in the same age group is 33%.1
The most common symptom of unicompartmental arthritis of the knee is pain confined to the affected compartment, which may be associated with swelling, effusion, instability, impingement, crepitus, stiffness and malalignment. Radiological findings of unicompartmental arthritis of the knee may include joint space narrowing, squaring of the femoral condyle, subchondral sclerosis, intercondylar spurring, joint line osteophytes, and varus or valgus malalignment of the affected limb.3Symptoms may not be sufficiently severe to require invasive treatment, but if they are it is an attractive option to merely address the damaged part of the joint, leaving the normal compartments alone. It seems logical to only treat the damaged part of the knee joint, leaving the good-quality ligaments and articular cartilage in place. It also seems possible that replacing the damaged part of the joint where bone loss and collapse can be anticipated might slow down or even prevent the development of multicompartmental arthritis.4
With the introduction of the minimally invasive technique and improved survival rates, unicompartmental knee replacement has become increasingly popular. However, controversy remains with regard to its role in knee reconstruction. Clinical research in orthopaedics is based on the assumption that the results of clinical data of a study population reflect the population with the same disease as a whole and the orthopaedic society as a whole. More reliable interpretation of the results on unicompartmental knee arthroplasty requires knowledge of the history of and basic information on unicompartmental knee arthroplasty. In this thesis these different aspects of unicompartmental knee arthroplasty are highlighted in chapters 2, 3 and 4.
The specific aetiology of degenerative osteoarthritis of the knee is unclear, but when it occurs in an active patient the process is generally progressive.5, 6In most cases, articular cartilage degeneration in the knee probably results from high-impulse loading occurring upon heel strike-7or trauma.8, 9The degenerative process results in deterioration of articular cartilage and symptomatic degenerative arthritis. Clinical symptoms include joint stiffness, pain and dysfunction. The principal problem for most patients is pain.
The medial compartment is more frequently affected, given the heavier loading under physiological conditions, and progression to the lateral compartment is uncommon.2, 6Varus deformation progresses, which accelerates the degenerative process by overloading the medial compartment. Angular malalignment on its own may contribute to the development and progression of unicompartmental arthritis by overloading.3Coronal plane deformity causes varus or valgus.
In order to appreciate deformity in the arthritic knee, it is important to understand alignment in the normal knee. The mechanical axis of the weight-bearing lower limb is a straight line, which runs from the centre of the femoral head through the centre of the knee joint (or just medial to it), through to the centre of the talus at the ankle joint. The anatomical axis is a line which runs down the centre of the femoral shaft to the centre of the knee and then to the centre of the talus. The angle sub-tended by those two axes is in a range of 4-8 degrees (average 6 degrees) of valgus in normal individuals, usually towards the lower end of range in males and the upper end in females.10
Depending on the different pathways of the arthritic process, meniscal disruption may occur or ligamentous insufficiency may develop with concomitant femorotibial subluxation. Progression of osteoarthritis to the lateral compartment is usually associated with anterior cruciate ligament rupture, leading to a loss of the normal sliding and rotation within the knee joint during flexion and extension.11When the ACL is intact, the orderly flexion-extension movement of the femur on the tibia in the sagittal plane is preserved, maintaining the separation of the flexion areas from the extension areas. Progressive loss of bone causes increasing varus deformity in extension but not in flexion. In flexion the medial collateral ligament is stretched to its normal length. Failure of the ACL allows posterior subluxation of the femur on the tibia, and offers sufficient explanation for the progression from anteromedial arthritis to posteromedial disease.
Progressive disintegration of the joint results from complete loss of cartilage substance, with progressive narrowing of the joint space. The ensuing loss of joint height in flexion allows the medial collateral ligament to remain shortened throughout the full range of knee movement, resulting in permanent contracture of the ligament. Rupture of the ACL is associated with the development of fixed varus deformity.11
This description of the progression of osteoarthritis explains both the association between an intact anterior cruciate ligament and preservation of the lateral compartment and the association between rupture of the ACL and the development of a fixed varus deformity12.
Early treatment of unicompartmental arthritis of the knee may improve the functional well-being and long-term outcome in patients. Nonoperative treatments to reduce the pain associated with joint inflammation include weight loss, anti-inflammatory or analgesic medication, intraarticular injections, periarticular muscle strengthening and stress offloading with braces or heel wedges.3
The reduction of joint reaction forces and symptoms of degenerativearthritis by a decrease in body mass is a fundamental conceptin the management of arthritic joints.13Obesity is an independentrisk factor for the development of osteoarthritis in the knee,14and this association is higher for women than for men.15Women over the age of 50 with malalignment have a higherprevalence of degenerative arthritis of the knee than do age-matchedcontrol subjects in the general, nonaffected population.16Weight loss by obese women decreases the risk of developing degenerative arthritis. A weight loss of 5.1 kg over tenyears has been shown to decrease the risk of degenerative arthritisby >50%.17
Nonsteroidalanti-inflammatory medications are the most commonly used drugsfor treatment of degenerative arthritis of the knee and otherjoints. These drugs, which inhibit cyclooxygenase 1and 2, have analgesic and anti-inflammatory properties, butthey can be associated with gastrointestinal and other sideeffects. In a short-term clinical trial in which acetaminophenand ibuprofen, in analgesic and anti-inflammatory doses, werecompared as treatments for symptomatic arthritis of the knee,the efficacy of the two drugs was equivalent.18When nonsteroidalanti-inflammatory drugs are used chronically, it is importantfor patients to have medical monitoring of the hepatic, renal andgastrointestinal systems.19Specific cyclooxygenase-2 inhibitorshave demonstrated clinical efficacy in the treatment of symptomaticarthritis of the knee, with decreased gastrointestinal and renalside effects. However, specific cyclooxygenase-2 inhibitorsare more expensive, and risk-benefit and cost-benefit analysesmust be completed to better define the role of these agents.19Nutritional supplements (so-called nutriceuticals), such asglucosamine and chondroitin sulfate, have been touted as chondroprotectiveagents. Double-blind, placebo-controlled, randomised trialshave shown that glucosamine is mildly effective for relievingpain associated with degenerative arthritis.20-23In astudy by Reginster et al.,24212 patients with osteoarthritisof the knee were randomised to a glucosamine or a placebo treatmentgroup. After three years of treatment, the glucosamine grouphad less joint-space narrowing and improved WOMAC (Western Ontarioand McMaster University Osteoarthritis Index) scores when comparedwith the placebo group. However, we are not aware of any long-termstudies demonstrating beneficial effects of glucosamine on thearticular cartilage of an arthritic joint.20Some patientswith unicompartmental arthritis of the knee report improvementwith use of topical analgesics (e.g. methyl salicylate, capsaicinand nonsteroidal creams) as either adjunctive treatment or monotherapy.13
Acute exacerbations of degenerative arthritis of the knee presentingpain, swelling and effusion can be treated with aspirationof the knee joint and intra-articular injection of a corticosteroidpreparation. Corticosteroid injections are frequently combinedwith a local anaesthetic medication and canprovide short-term symptomatic relief. However, theseinjections can increase the risk of damage to the articularcartilage of the injected knee joint, and they should not berepeated more than three or four times a year.25
Exercise, as an adjunct to weight reduction, has value in thetreatment of an arthritic knee. Stretching to prevent contracture,maintain range of motion and increase muscle strength and dynamic stability of the knee can reduce symptoms associatedwith an arthritic knee.26Quadriceps muscle weakness is commonamong patients with degenerative arthritis of the knee and maybe a risk factor for this disease. Patient education programsand supervised fitness and walking sessions have been shownto improve functional status without worsening the symptomsof osteoarthritis of the knee.27
Three types of knee braces are commercially available for the treatmentof a knee with degenerative arthritis: compression knee sleeves,supportive knee braces and unloading knee braces. Polypropylene,neoprene or elasticised knee sleeves may minimise swellingand provide a feeling of increased support and warmth aboutthe knee without changing limb alignment, joint stability ormechanical function. Some patients report a feeling of securitywith a knee sleeve, possibly because of enhanced proprioceptivefeedback.26Supportive knee braces include hinged braces(for varus-valgus instability), anterior cruciate insufficiencybraces (for anteroposterior and rotatory instability) and patellofemoralbraces (for patellofemoral malalignment or instability). Unloadingbraces are designed to apply a varus or valgus force at theknee and relieve pain during activity by distracting the jointspace of the involved compartment during weight-bearing andactivity.28Heel and sole wedges can realign the foot 5° to 10°in either the varus or the valgus plane. With a lateral wedgeand insole, the shift in alignment reduces medial joint-spaceloading.29Keating et al.30evaluated 121 knees with medialunicompartmental arthritis in 85 patients who weretreated with a lateral heel and sole wedge. Sixty-one of the121 knees had a good or excellent result after four to 24months of treatment. Knees with all grades of arthritic involvementshowed improvement. Patients with stage-II disease accordingto the modified Outerbridge classification31improved the most.
When nonoperative treatment of osteoarthritis of the knee fails to relieve pain and knee function is compromised, operative intervention is warranted. Surgical choices include arthroscopic debridement, joint reconstruction, or both. Joint reconstruction choices include osteotomy and knee replacement. Joint replacement can be unicompartmental or total.3
Arthroscopic debridement has long been considered an effective alternative in the treatment of osteoarthritis of the knee.32-34However, a recent study by Mosely and colleagues called its value into question.35They compareda placebo group with a group treated with arthroscopic jointlavage and another group treated with arthroscopic debridement.All three treatment groups had a decrease in symptoms up totwo years after intervention. This study was confined to oldermen in a Veteran's Administration Hospital. The extent of thearthritic involvement of the knee (in one, two or three compartments)was not documented. Patients were not stratified accordingto degree of malalignment, body weight or type of symptoms.The authors concluded that arthroscopy of a knee with degenerativearthritis may not be indicated when there is only pain in theabsence of other symptoms (such as catching, clicking, lockingor giving way). Furthermore, they suggested that a decreasein symptoms after arthroscopy may be associated with a placeboeffect. There have been several other retrospective studies of arthroscopictreatment of degenerative arthritis of the knee, with unfavourable results.36-38
The rationale for an osteotomy is based on the premise that excessive varus or valgus deformity leads to harmful stresses on the articular cartilage, which in turn lead to osteoarthritis. As long as overcorrection is avoided, corrective osteotomy ofthe knee is associated with biological improvement of damagedarticular cartilage with maintenance of articular cartilagein the least degenerated compartment.39-48
Unicompartmental knee arthroplasty is a potentially attractive alternativeto tibial osteotomy or total knee arthroplasty in selected osteoarthriticpatients. Traditionally, unicompartmental knee arthroplasty hasbeen reserved for patients with unicompartmental arthritis who havea sedentary lifestyle and are older than 60.However, there has been recent interest in performing this procedurein patients younger than 60 as an alternative to tibialosteotomy or total knee arthroplasty.49
Total knee replacement has been shown to have durable and predictable resultsin elderly patients, providing pain relief, improving functionand correcting deformity. The possibility ofmultiple revisions due to loosening or wear initially discouragedthe widespread use of total knee arthroplasty in young patients withdegenerative arthritis.37These concerns arose from the poorresults observed in young patients who had had a total hip arthroplasty. However,the early results of total knee arthroplasty in young patients didnot reflect the experience with total hip arthroplasty, and preliminary reports often included many patients with rheumatoid arthritisor juvenile rheumatoid arthritis. On the basisof this initial success, the indications for total knee arthroplastywere eventually expanded to younger patients with osteoarthritis. As the indications continue to expand, the decision to proceedwith total knee arthroplasty in young, active patients needsto be individualised after careful consideration of alternatives.49
Unicompartmental knee replacement for unicompartmental knee arthritis has been available for many years. McKeever first introduced the concept of resurfacing the diseased compartment of the knee in the 1950s, when he introduced a metal tibial plateau prosthesis.50This idea was expanded in the 1970s, when Marmor added modularity to an all-polyethylene tibial component articulating with a metal polycentric femoral runner.51In 1988, Marmor published disappointing results after 10-13 years.52, 53In about 20% of the cases there was wear of the polyethylene or a symptomatic arthritis of the contralateral compartment. The Swedish joint replacement registry also published unsatisfactory results, mainly because the unicompartmental knee prosthesis was also placed for chronic inflammatory conditions.54Other authors also indicate revision operations in 20-28% of cases.55, 56For this reason, unicompartmental knee prosthesis became less popular in the 1980s. Subsequent improvements have greatly benefited unicompartmental knee arthroplasty, and recent reports of unicompartmental knee arthroplasty have been more encouraging.57-60The improved results have been attributed to improved prosthetic design, improved surgical technique and precise patient selection.
To clarify the role of unicompartmental knee replacement, it should be compared with the alternative procedures, such as proximal tibial osteotomy and total knee replacement. Unicompartmental knee arthroplasty and upper tibia osteotomy have been used to treat unicompartmental arthritis in similar patients. For some, corrective tibial head osteotomy is even losing its position as the golden standard.61-64A comparative study by Broughton into the results of corrective tibial head osteotomy as against those of the unicompartmental knee prosthesis show respectively 43% and 76% good results.63Late degeneration in the lateral compartment was not seen after placement of a unicompartmental knee prosthesis. Weale and Newman report better function and longer survival of the unicompartmental knee prosthesis compared to tibial head osteotomy.62More studies show a quicker full recovery, easier rehabilitation, fewer perioperative complications and better long-term results after a 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.65
The results of long-term follow-up of unicompartmental knee arthroplasty demonstrate predictably good results, comparable with those of total knee replacement.66-68Unicompartmental knee arthroplasty has multiple advantages over total knee arthroplasty. The surgical approach and surgical dissection can be considerably less extensive for unicompartmental knee arthroplasty,69, 70and there is a decreased transfusion requirement.71There is a lower implant cost.72The range of motion is usually greater,71, 73and the shorter postoperative recovery limits hospital stay.74Additionally, there is little compromise in doing a conversion of a unicompartmental knee arthroplasty to a total knee replacement, and it is less difficult than revision of a total knee replacement.75, 76
Approximately 6% of patients with arthritis of the knee are eligible for unicompartmental knee arthroplasty.77Since only one of the three compartments is to be replaced, it is clearly necessary for the disease to be predominantly confined to that compartment. The ideal candidate has low activity demands and a stable knee.
Absence of the anterior cruciate ligament is a contraindication because its presence makes the combined rolling and sliding at the femoral and tibial interfaces possible, which may yield near-normal joint kinematics and mechanics.78The flexion deformity should be less than 15 degrees. Unicompartmental knee arthroplasty has only a limited ability to improve flexion deformity. There should be no varus or valgus alignment or minimal malalignment that can be passively corrected on examination.3Patellofemoral osteoarthritis is not a contraindication.79There is no influence on the clinical outcome, there is no progression seen after a mobile-bearing unicompartmental knee arthroplasty, and patellofemoral osteoarthritis has not been a reason for revision,80although it may be a contraindication when patellofemoral symptoms are present preoperatively.81, 82Patients with inflammatory diseases such as rheumatoid arthritis are not candidates for unicompartmental knee arthroplasty because of synovial involvement of the opposite compartment.83, 84Knee arthroplasty in rheumatoid arthritis is associated with progression of RA and loosening of the components.85
The introduction of minimally invasive techniques emphasises smaller skin and capsular incisions, limited violation of the extensor mechanism or suprapatellar pouch, better functional results, reduced morbidity and decreased rehabilitation time.95-100Nevertheless, with the minimally invasive technique86, 87the visual field is restricted; this makes unicompartmental knee arthroplasty a demanding procedure.65, 88The nature of the implant, careful patient selection, experience of the surgeon, instrumentation and surgical technique are very important in unicompartmental knee arthroplasty.65, 89Recent studies suggest that the results of unicompartmental knee arthroplasty compare well with the results of total knee arthroplasty in similar age groups.58, 90-93In addition, indications for the procedure are broadening with the newer techniques and designs.94It seems that by preserving the intact compartments of the joint and merely replacing the damaged areas, excellence should be achievable.
The purpose of this thesis is to evaluate the short- and mid-term results of the Oxford phase-III unicompartmental knee replacement in patients by independent surgeons. It makes sense to examine the position of unicompartmental knee replacement with its introduction in a community hospital. Chapters 5, 6 and 7 report on the outcome. In chapters 8 and 9 the pitfalls of the Oxford phase-3 mobile-bearing unicompartmental knee arthroplasty that may be avoided with awareness and careful surgical technique are mentioned.
Popliteal vascular injury and compartment syndrome of the lower leg are rare but critical complications of knee arthroplasty. Early diagnosis and treatment are of paramount importance in preventing the devastating consequences of these conditions. These complications are described and analysed in Chapter 10. Young patients with incongruity of the articular surface as a result of an osteochondritis dissecans lesion and advanced unicompartmental degenerative knee joint disease present a difficult and challenging therapeutic problem. In Chapter 10 we also report a case of unicompartmental knee replacement for osteochondritis dissecans of the medial condyle of the knee in a young adult.
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