Unicompartmental Knee Arthroplasty

Updated: Sep 22, 2017
  • Author: Nanne P Kort, MD, PhD; Chief Editor: Thomas M DeBerardino, MD  more...
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Unicompartmental knee arthroplasty (UKA) has had varying degrees of acceptance since its introduction several decades ago. [1, 2, 3, 4, 5, 6]  Frequent and early failures were cited in initial studies. Consequently, by the late 1980s, the popularity of UKA 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 UKA again increased, survivorship being comparable to that for total knee arthroplasty (TKA). Although in 1994, the Swedish Joint Replacement Registry reported a high percentage of poor results for UKA, these outcomes mainly reflected the fact that the surgery was performed on patients with chronic inflammatory arthritis. [7]

Interest in UKA was also stimulated by the introduction of the mobile-bearing form of the procedure. Goodfellow and O'Connor, along with others, published excellent long-term survivorship rates associated with this technique. [8]  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).

Subsequently, there was once again a heightened interest in UKA, 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 view UKA as preferable to high tibial osteotomy (HTO) in relatively young patients with medial compartment arthritis.

Theoretical advantages of the procedure include preservation of uninvolved tissue and bone, reduced operating time, better range of motion (ROM), improved gait, and increased patient satisfaction. With appropriate patient selection, careful surgical technique, and proper implant design, UKA can now be viewed as a procedure with reliable medium- to long-term success. [9, 10]

Because UKA is typically performed for medial osteoarthritis, this article focuses primarily on that condition.

For patient education information, see the Arthritis Center, as well as Knee Joint Replacement.



Careful patient selection is critical for UKA 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 ligament (ACL) is a contraindication; this ligament 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°. UKA with excision of osteophytes in the notch cannot correct moderately severe flexion contractures.

Ideally, it should be possible to flex the knee to 110°. This is important for the preparation of the femoral condyle during the operation.



Contraindications for UKA include the following:

  • 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 UKA)
  • Valgus deformity greater than 20° (lateral UKA)
  • Flexion less than 110°

UKA 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 UKA is rare, according to some studies. In the Swedish Registry, no UKAs required revision for patellofemoral problems. [7]

Murray et al reported that residual postoperative pain was independent of the state of the patellofemoral joint, and no knee surgery was revised because of patellofemoral problems. [11, 12, 13]  UKA 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 UKA. For more information, see Patellofemoral Joint Arthritis.

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 UKA than older individuals would. 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 UKA in younger patients. [14]


Technical difficulties and increased risk of complications are associated with obesity. However, obesity is not considered a contraindication particularly for the UKA mobile-bearing design. This is because a correlation has not been found between obesity and wear.


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 UKA) 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 Brumby et al indicated that crystalline arthropathy is a contraindication for unicompartmental replacement, Murray et al disagreed. [15, 11]


Technical Considerations

Complication prevention

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.



When nonoperative and, possibly, arthroscopic procedures fail, the surgeon may consider HTO, UKA, or TKA. UKA appears to result in better function, greater pain relief, less morbidity, and higher patient satisfaction than do HTO and TKA.

The long-term survival rate for UKA [16]  is higher than that for HTO and comparable to that for TKA. With strict indications, newer prostheses, and attention to surgical technique, UKA has become a valuable treatment for unicompartmental knee arthritis.

In a prospective study, Berger et al found that in a 3-month period following knee arthroplasty, no patients in the investigation who had undergone UKA required hospital readmission, compared with 9% of patients who received TKA. [17] In the study, the authors looked at 111 patients who had undergone primary knee arthroplasty, of whom 25 underwent UKA and 86 underwent TKA. Of the 111 patients, 104 (24 with UKA and 80 with TKA) met discharge criteria and were discharged directly to home.

Saenz et al evaluated the clinical and radiographic outcomes of the EIUS unicompartmental prosthesis and found it to be associated with higher revision rates than were metal-backed implants. [18] The implant survival rate was 89%, with 16 knees either revised or scheduled for revision. The reasons for revision included aseptic loosening of the tibial component, progressive symptomatic patellofemoral disease, and tibial component subsidence.

Unicompartmental knee arthroplasty vs high tibial osteotomy

Resurfacing methods are gaining popularity. Results comparing HTO with UKA favor the latter.

Broughton et al demonstrated good results in 76% of patients in a replacement group and in 43% of patients in an osteotomy group. [19]  ROM, speed of rehabilitation, and perioperative morbidity were significantly better for UKA, and no signs of late deterioration were present. Weale et al, after a 12- to 17-year follow-up period, also reported better function and longer survival in the unicompartmental group. [20]

Other publications have similarly shown more favorable results with arthroplasty. [21, 22]  The functional advantages of UKA over HTO have also been demonstrated by using gait analysis, with patients displaying a more normal gait and better stair-climbing ability after UKA than they did after HTO.

If a revision to TKA becomes necessary, the results are now believed to be generally better if the revision occurs after a failed UKA than they are following a failed HTO. (Previous HTO is a contraindication for UKA.)

Unicompartmental knee arthroplasty vs total knee arthroplasty

In the late 1980s, UKA waned in popularity, largely because of problems with patient selection, operative technique, and polyethylene wear. Later, as understanding of the procedure and the associated prostheses improved, the long-term results of UKA became comparable to those of TKA. [23, 24]

Functional outcome with UKA is superior to that with TKA, with the former providing better ROM and ambulatory function. Laurencin et al found that UKA also results in less pain, more stability, and better stair-climbing ability than does TKA. [25]  In addition, the cost of UKA is about 57% that of TKA.

Dalury et al found little or no difference in outcome between patients who received TKA and those who received UKA, except for slightly better ROM with UKA than with TKA (123º±9º and 119.8º±7º, respectively). Of the 23 patients in the study, 11 expressed no preference for either knee and 12 preferred the unicompartmental knee; no patient preferred the total knee. [23]