Unicompartmental knee arthroplasties (UKA) or replacements (UKR) are orthopedic procedures where either the medial or lateral articular surfaces of the knee are replaced by prostheses 1. This differs from patellofemoral arthroplasties, which replace the patellar and trochlear groove articular surfaces. Globally, UKAs make up ~7.5% (range 5.2–9.8%) of arthroplasties performed with medial replacements the vast majority (~97%) 1,2.
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History
UKAs date back to the 1940s and its popularity has waxed and waned over time 3 as early implant failures led to a high revision rate. Reasons include early aseptic loosening, polyethylene wear, and design error, which has led to a subsequent lack of surgeon uptake 4.
In the modern-day, with the development of ultra-high molecular weight polyethylene, UKA has been successfully used in appropriately selected patients 5. With the advent of robotic surgery, proponents of this technique have argued that it further exemplifies the benefits of this minimally invasive, bone, ligament and soft tissue-sparing procedure 6.
Indications
Indications to perform a UKA include 7:
single-compartment arthritic disease
intact anterior cruciate ligament
intact peripheral ligaments
correctable coronal deformity <15 degrees
flexion contracture <15 degrees
range of motion at least 90 degrees
Classically taught indications such as age >65 years, weight <82 kg and sedentary life style are no longer considered factors that affect decision making for UKA, although they can affect survivorship of implant 7.
Comparison
Compared to TKR 8:
shorter operation with faster recovery and reduced hospital stay
fewer serious medical complications with less blood loss and lower deep infection rates
better functional and PROM scores
quicker return to sport and work
higher revision rates
Compared to osteotomies 5:
worse range of movement
quicker post-operative recovery
Contraindications
Contraindications include 7:
significant or non-correctable varus or valgus deformity
Previously described contraindications such as anterior knee pain, chondrocalcinosis, and high BMI are no longer considered as absolute reasons not to proceed with a UKA 7.
Prosthesis design
The design of the UKA has undergone significant change since it was first introduced by Campbell using interpositional vitallium plates in the medial compartment of the knee for arthritis 3.
The design of the UKA consists of a femoral component, a tibial component and a meniscal bearing spacer. The different choices of the design available include the bearing design, cementing and material:
tibial component: metallic plate with pegs/keel or an all-polyethylene tibial component
femoral component: metallic hemispheric anatomic conforming femoral component
meniscal bearing spacer component: polyethylene component
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there are two types of spacer fixation
fixed-bearing: less conforming articular surface, increased point loading, low risk dislocation, less stringent implant parameters, higher polyethylene wear 9
mobile-bearing: high conforming articular surface designed to replicate natural joint kinematics, reduced stress loading, lower polyethylene wear, more stringent implant parameters with malpositioning leading to earlier failure, high risk of polyethylene dislocation 9
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there are three cementing options
cemented: early bone-implant fixation, more reliable in osteoporotic bone, risk of aseptic loosening, extended surgical time 10
cementless: bony ingrowth into prothesis more reliable fixation method, no cement particular debris, shorter surgical time 9
mixed: typically cemented tibial prosthesis and cementless femoral prosthesis; no long-term data 11
Radiographic features
Plain radiograph
AP projection
The coronal angle of the femoral component is made between the implant long axis and the femoral anatomic axis. A mobile-bearing UKA has an acceptable range of ±10 degrees for the femoral component, lower for fixed-bearing UKA 12.
The coronal angle of the tibial component, is made between a perpendicular line drawn along the base of the tibial tray and the tibial anatomic axis. A neutral angle is 0 degrees and the varus or valgus alignment angle is relative to this. A mobile-bearing UKA has an acceptable range of ±5 degrees for the tibial component 12.
The tibial component should be visualized just medial to the apex of the tibial spine, with slight 0-2 mm of overhang on the medial border of the tibia 12.
Lateral projection
The sagittal angle of the femoral component is made between the axis of the central peg of the component and the femoral anatomic axis. Neutral angle is considered 0 degrees. A mobile-bearing UKA has an acceptable range of ±10 degrees of flexion/extension for the femoral component 12.
The sagittal angle of the tibial component is made between the a line drawn along the base of the tibial tray and the tibial anatomic axis. Neutral angle is considered 7 degrees with a posteroinferior slope. A mobile-bearing UKA has an acceptable range of ±5 degrees of tilt for the tibial component 12.
The femoral component should overhang the bone on the lateral proximally by 2-3 mm 12.
Lyson Schuss view
The Lyson Schuss view offers greater accuracy in determining joint space loss, to aid in determining contralateral compartment arthritis in the setting of a UKA, and need for a revision procedure 13.
CT
Metal artifact reduction reconstruction can assist in answering the following clinical questions:
implant assessment, e.g. femoral or tibial component overhang, component rotation, polyethylene wear
assessment for fractures or microfractures
assessing for osteolysis, i.e. aseptic loosening, periprosthetic infection
Nuclear medicine
A bone scintigraphy may be useful in the case of a painful UKA to look for radiotracer uptake and assess for potential aseptic loosening.
Complications
Potential complications include 15:
aseptic loosening
hardware failure, e.g. polyethylene wear, polyethylene bearing dislocation
accelerated arthritis of the contralateral compartment
impingement
arthrofibrosis
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