Medial Unicompartmental Knee Arthroplasty with a Mobile-Bearing Implant
Authors: Bonano John C. , Barrett Andrew A. , Amanatullah Derek F. 
Published: April 2021

Background:
For patients with isolated medial knee arthritis, unicompartmental knee arthroplasty (UKA) is an appealing surgical option. Compared with total knee arthroplasty (TKA), UKA is less invasive, preserves more native bone stock, has lower perioperative morbidity and mortality
1,2
, allows for quicker recovery
1,2
, and has higher patient satisfaction
2,3
. It also allows for more dynamic proprioception and postural control, allowing for function more similar to a native knee
4
. Proper surgical technique and patient selection are critical to the growing success of UKA.

Description:
The general principles of UKA are to remove and replace the diseased femoral and tibial joint surfaces while restoring limb alignment. The key steps of this procedure are demonstrated in the video article: (1) patient evaluation, (2) patient positioning, (3) surgical exposure, (4) tibial guide placement and resection, (5) femoral guide placement and resection, (6) knee balancing and implant trialing, (7) preventing impingement, (8) final tibial preparation, (9) final component placement, and (10) closure and postoperative course.

Alternatives:
Patients should first exhaust all nonoperative treatments before considering UKA. For those patients who do not meet the appropriate indications for UKA, TKA is the most acceptable alternative treatment. In active patients <55 years old without bone-on-bone osteoarthritis, a high tibial osteotomy may be considered. When considering UKA, options include both mobile-bearing and fixed-bearing designs.

Rationale:
The potential advantages of UKA over TKA make it an attractive option for the appropriate patient. Recently, indications for UKA have expanded to include (1) isolated noninflammatory bone-on-bone arthritis in the medial compartment; (2) retained full-thickness cartilage in the lateral compartment, best visualized on valgus-stress radiographs; (3) a functionally normal medial collateral ligament; and (4) a functionally normal anterior cruciate ligament
5
. Mobile-bearing UKA designs consist of a dual articulation between the concave polyethylene insert and the metallic tibial and spherical femoral components, which allows for distribution of contact forces over a large surface area
6
. In contrast, fixed-bearing designs have a flat polyethylene insert that does not conform to the femur. Current data do not demonstrate a clear difference in clinical performance between the 2 bearings. However, a recent systematic review has shown lower polyethylene wear rates in mobile-bearing implants
7
.

Expected Outcomes:
The survivorship of mobile-bearing UKA ranges from 85% to 98% at 10 years
8–12
and from 79% to 94% at 15 years
8,13,14
. The most common indications for revision include aseptic loosening (36%), progression of osteoarthritis (20%), and unexplained pain (11%)
7
. Bearing dislocation is a unique complication for mobile-bearing UKAs, with reported rates between 0.3% to 4.2%
6,10,12
.

Important Tips:

Always inspect all compartments of the knee for evidence of substantial degenerative changes prior to proceeding with UKA. If there is advanced arthritis in other compartments, the decision should be made to proceed with TKA instead. Therefore, preoperatively, all patients indicated for UKA should also sign a consent form for TKA, and TKA implants should be available in case they are needed.When assessing bearing size, it is important that there is no axial loading through the heel to properly assess the flexion gap. Intraoperatively, this can be done by elevating the thigh so that the foot hangs freely. The trial spoon should fit into the joint without substantial resistance or effort.Spigots do not reference the end of the bone. Rather, they reference the end of the spigot hole that was drilled using the initial 0 spigot.Spigot number represents the total thickness of additional bone that will be removed from the first mill cut.Confirm the amount of bone that was resected by inspecting the small rim of bone that remains around the spigot hole after milling.