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 inflammatory arthritis ACL deficiency absolute contraindication for mobile-bearing UKA and lateral UKA controversial for medial fixed-bearing 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