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Updated: Jun 22 2023

Unicompartmental Knee Replacement

  • summary
    • Unicompartmental Knee Arthroplasty is a surgical option for knee arthritis when only one compartment of the knee is involved.
    • The procedure can be performed for isolated medial compartment, isolated lateral compartment or isolated patellofemoral osteoarthritis. 
    • The most common reasons for conversion to a total knee arthroplasty are the progression of osteoarthritis and aseptic loosening.
  • Epidemiology
    • Incidence
      • 5% of surgeries where knee arthroplasty is indicated are unicompartmental knee replacements
    • Anatomic location
      • medial compartment is most common
  • Types of implants
    • Fixed-bearing
      • historical standard of care
    • Mobile-bearing
      • pros
        • weightbearing through the meniscus increases conformity and contact without increasing constraint
        • decrease in wear pattern
        • excellent survivorship out to the second decade
      • cons
        • technically demanding
        • bearings can dislocate
  • Advantages
    • Compared to TKA
      • faster rehabilitation and quicker recovery
      • less blood loss
      • less morbidity
      • less expensive
      • lower rates of PJI, wound complications
      • preservation of normal kinematics
        • theory is that retaining ACL, PCL and other compartments leads to more normal knee kinematics
      • smaller incision
        • less post-operative pain leading to shorter hospital stays
    • Compared to osteotomy
      • faster rehabilitation and quicker recovery
      • improved cosmesis
      • higher initial success rate
      • fewer short-term complications
      • lasts longer
      • easier to convert to a TKA
  • Indications
    • Indications
      • controversial and vary widely
      • as an alternative to total knee arthroplasty or osteotomy for unicompartmental disease
      • classicaly reserved for older (>60), lower-demand, and thin (<82 kg) patients
        • 6% of patient's meet the above criteria with no contraindications
      • new effort to expand indications to include younger patients and patients with more moderate arthrosis
    • Contraindications
      • ACL deficiency
        • absolute contraindication for lateral UKA
        • controversial for medial fixed-bearing and for mobile-bearing UKA
      • fixed varus deformity > 10 degrees
      • fixed valgus deformity >5 degrees
      • restricted motion
        • arc of motion < 90°
      • flexion contracture of > 5-10°
      • previous meniscectomy in other compartment
      • tricompartmental arthritis (diffuse or global pain)
      • younger high activity patients and heavy laborers
      • grade IV patellofemoral chondrosis (anterior knee pain)
  • Technique
    • Procedural tips
      • avoid overcorrections
        • undercorrect the mechanical axis by 2-3 degrees
        • overcorrection places excess load on unresurfaced compartment
      • remove osteophytes (peripheral and notch)
      • resect minimal bone
      • avoid extensive releases
      • avoid edge loading
      • prevent tibial spine impingement with proper mediolateral placement
      • avoid making a varus tibial cut which increases the chance for loosening
      • use caution when placing the proximal tibial guide pins to avoid stress fractures
      • correct varus deformity to 1-5 degrees of valgus
  • Complications
    • Aseptic loosening
      • most common cause of early failure (5 years) at somewhere between 25%-45.3%
    • Stress fractures
      • always involve tibia
      • associated with high activity and patient weight
      • clinically there will be a pain free interval followed by spontaneous pain with activity
      • blood commonly found on joint aspiration
      • risk factors
        • penetrating posterior tibial cortex with guide pin, placing guide pin medial in periphery, re-drilling for guide pin, and under-sized tibial component
    • Intra-operative fractures
      • associated with forceful impacting of implant
  • Outcomes
    • Fixed-bearing
      • 1st decade results
        • 10-year survivorship from studies done in 1980s and 1990s ranges from 87.4% to 96%
        • the standard faliure rate in the first decade is 1%
      • 2nd decade results
        • rapid decline in survivorship ranging from 79% to 90%
    • Mobile-bearing
      • excellent clinical results with 15-year survivorship reported at 93%
    • Long-term results
      • lateral compartment arthroplasties have equivalent results to medial
      • revision rates are worse than total knee revision rates
      • Patellofemoral arthroplasty (PFA) has good outcomes for isolated patellofemoral arthritis
        • Previous generation designs (i.e. inlay style) exhibited high rates of patellar instability
        • Newer generation designs (i.e. outlay style) replaces entire anterior trochlear surface and minimizes risk of patellar instability
        • Long term mode of failure remains progression of tibiofemoral arthritis
      • causes of late failure (>5 years)
        • progress of osteoarthritis (idiopathic, over-correction, more common with mobile-bearing)
        • component failure (overload due to under-correction)
        • component loosening (common in fixed-bearing)
        • patella impingement on femoral component (patella pain)
        • polyethylene wear
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