Unicompartment Knee Replacements

History

  • The first cemented metal on polyethylene UKR was used by Marmor in 1972.
  • He obtained better results with resurfacing of the lateral compartment. The Oxford replacement followed 10 years later.
  • Theoretical advantages of unicompartmental knee replacement
  1. Preserves undamaged compartments
  2. Preserves cruciates thereby preserving kinematics
  3. Cheaper
  4. Less morbidity
  5. Shorter stay in hospital
  6. Less blood loss & less need for blood transfusion
  7. Some theoretically easier to re-surface. However, Barrett & Scott reported that significant bone grafting or augmentation was necessary in 45% of revisions, & Padgett & Insall found significant bone defects in 76% at revision. Modern prostheses may have less problems at revision

Indications

  • Range of Motion
    • less than 15° fixed flexion
    • flexion arc of at least 90°
    • Oxford surgical technique recommends that the knee be able to flex to at least 120°.
  • Varus or valgus deformity needs to be passively correctable
  • Age less than 60 years with low demand on the knee
  • Weight less than 82 kg ideal
  • Pain should be minimal at rest as this indicates inflammatory component of the disease & generalised inflammation is a contraindication
  • Movement of at least 90° & any flexion contracture should be < 5o
  • Angular deformity should be < 15o (10° varus to 15o valgus) & be passively correctable to neutral
  • Both cruciates should be intact
  • Final decision to proceed after inspection of the joint surfaces
  • Patello femoral joint pain & chondrocalcinosis are relative contra-indications to proceed

Contraindications

  • Inflammatory & crystalline arthropathies
    • results are inferior to those obtained with TKR.
  • Age
    • Kozinn & Scott advocate patients older than 60.
  • Weight > 82kg
  • Inflammatory arthropathy
  • Dynamic instability
  • Cruciate deficiency of the knee
  • FFD greater than 15 o Flexion of less than 90°

Oxford Study

JBJSB November 1998, Murray & Goodfellow

  • This was a 10-year survival study looking at the Oxford prosthesis for antero-medial Osteoarthritis.
  • Oxford prosthesis
    • fully congruent
    • mobile polyethylene bearing free to move between chrome cobalt cemented femoral & tibial components
    • first used in 1982
    • femoral component is four sizes
    • 5 tibial component sizes
    • polyethylene thickness ranges from 3.5 to 11.5mm in 1mm increments.
  • This study looked at 143 knees, with an average followup of 7.6 years.
  • cumulative survival rate was 98% with a worst case of 97%.
  • average age of the patients at operation was 71.

They had strict inclusion criteria:

  • THE ACL HAD TO BE NORMAL.
  • final decision about performing a UKR or TKR was made after inspection of the ACL.
  • A normal ACL is defined as one that retains its synovial covering & has no longitudinal splits
    • Previous work by the same authors had determined that the rate of failure without a functioning ACL was 10 times higher
  • Preop valgus stress views demonstrated full thickness of articular cartilage on the lateral compartment & correctability of deformity.
  • (Coolican stated at trial exam that if subluxation is correctable this allowed a unicompartmental knee replacement to be performed. This is a controversial position.
  • Dee & Hurst’s textbook states “any subluxation of the tibiofemoral articulation is a contraindication to surgery even with acceptable coronal alignment”))
  • They excluded patients who had had previous HTO

Things that did not influence their decision:

  1. Obesity (Kozinn & Scott feel patients should be less than 180lb)
  2. Chondrocalcinosis
  3. State of patellofemoral joint (Argenson says he is considering only doing TKR if the patellar is significantly involved)
  4. Osteophytes or full thickness cartilage loss on the medial margin of the lateral femoral condyle
  • They did not perform soft tissue releases.
  • Other surgeons have not matched these results but have probably had looser indications. (Lewold et al from Sweden reported a 90% 5 year survival rate, with the commonest failure being dislocation of the prosthesis).
  • authors state that incongruent surfaces require polyethylene at least 8mm thick. Their congruent surface provides an area of contact of about 6 square cm rather than point loading. Their average annual rate of penetration was 0.026mm.
  • Other results
  • 87% pain free at 10-13 years (Marmor 1988); 70% satisfactory results. Performed between 1972-1976, assessing 60 of 87 knees.
  • Marmor commented that the morbidity & complications are much less than with high tibial osteotomy. He felt that good candidates for the procedure were patients older than 50 years of age with a low activity level & normal weight.
  • Miller-Galante 94% survival rate at 10 years in 2 studies: Argenson from France 2002, Berger 1999.
  • General comments on UKR
  • There should be no attempt to correct deformity using the prosthesis. Correction leads to degeneration of the other compartment. If anything, under correction is preferred, to around 3° varus.
  • If a patient has a valgizing HTO there is a tendency towards excessive valgus alignment if a medial unicompartmental knee replacement is performed.
  • results of lateral UKR have been poor. If doing a lateral UKR aim for a neutral tibial femoral angle.
  • Unicompartmental vs. total knee replacement
  • Newman et al JBJSB Sept 1998
  • Randomized controlled trial between St Georg Sled & Kinematic TKR (with all cemented components & routine patellar resurfacing) with 102 knees followed for 5 years; patient age was 69 with a predominance of females.
  • Their criteria for inclusion were: unicompartmental Osteoarthritis; intact cruciates; flexion deformity & varus valgus deformity of less than 15°
  • Demonstrated superiority of the UKR immediately postop & at 5 years. UKR patients left hospital faster, had fewer DVTs, fewer MUAs, better initial ROM. There were more patients with excellent results in the unicompartmental group. At five years, 69% of UKR patients had more than 120° of flexion compared with only 17% of TKR patients.
  • They stated that there had been previous retrospective & prospective non-randomized trials which also showed superiority of UKR over TKR in appropriate patients.
  • Note that the St Georg Sled prosthesis has an 88% 10 ysr which is similar to that of the Kinematic TKR.