Unicompartmental Knee Replacement - Recon

Introduction

Surgical option for knee arthritis when only one compartment of the knee is involved
Epidemiology
- 5% of surgeries where knee arthroplasty is indicated are unicompartmental knee replacements
- 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
  - 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

Technique

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