Unicompartmental knee arthroplasty

Introduction

Unicompartmental Knee Arthroplasty (UKA) or Unicompartmental Knee Replacement (UKR) is an orthopaedic procedure where a single articular surface of the femorotibial joint is replaced by both tibial and femoral prosthetic components ¹. The term UKA or UKR is reserved to describe the replacement of the medial or lateral compartments of the femoral and tibial articular surfaces. This is distinct from the patellofemoral replacement that replaces the patellar and trochlear groove articular surfaces.

Incidence

• Across the world, UKA made up between 5.2 – 9.8% of arthroplasties performed in 2022 ^{1, 2}.

• Medial sided UKA make up 97.3% of total UKA operations, vastly more commonly performed than lateral UKA ¹_.

History

• UKA have dated back to 1940s, but has never cemented itself as regular in the line-up of treatments for knee joint arthritis.

• The use of UKA has been selectively used with varying levels of success and its popularity has waxed and waned ³.

• This is due to early implant failures leading to a high revision rate. The reasoning for this is due to a variety of reported reasons including early aseptic loosening, polythene wear and even a design error, which has led to a subsequent lack of surgeon uptake ⁴.

• In the modern-day, with rapid advancement of technology and the development of Ultra-High Molecular Weight Polythene, UKA has been selectively used for the “right” patient successfully 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

• faster recovery

• shorter operation

• reduced hospital stay

• better functional scores

• better PROM scores

• lower deep infection rates

• fewer serious medical complications

• increased knee range of movement

• quicker return to sport and work

• less blood loss

• higher revision rates

Compared to osteotomies: ⁵

• higher functional scores but did not meet minimal clinically important difference (MCID)

• lower pain profile but did not meet MCID

• worse range of movement

• quicker post-operative recovery

Contraindications⁷

Contraindications include⁷:

• infection

• ACL deficiency

• significant or non-correctable Varus or Valgus deformity

• tricompartmental arthritis

Previously described contraindications such as anterior knee pain, chondrocalcinosis and large 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 current 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 the material.

• Tibial component: metallic plate with pegs/keel or an all-polythene tibial component.

• Femoral component: metallic hemispheric anatomic conforming femoral component.

• Meniscal bearing spacer component: polythene component.

Spacer fixation ⁹

• Fixed bearing

  • Less conforming articular surface

  • Increased point loading

  • Lower risk of bearing dislocation

  • More forgiving with greater room to tolerate surgical error

  • Higher polythene wear

• Mobile bearing

  • Higher conformity

  • Designed to replicate natural joint kinematics

  • Less stress loading

  • Less likelihood of polythene wear

  • Less forgiving than Fixed Bearing

  • Malpositioning of implant can lead to earlier failure

  • Higher risk of polythene dislocation

Cementing

• Cemented ¹⁰

  • Early bone-implant fixation

  • More reliable in osteoporotic bone

  • Risk of aseptic loosening

  • Extended surgical time

• Cementless ⁹

  • Bony ingrowth of prosthesis is a more reliable fixation than cement

  • Risk of no/minimal bony ingrowth

  • No cement particulate debris

  • Shorter surgical time

• Mixed ¹¹

  • Typically cemented tibial prosthesis and cementless femoral prosthesis

  • Short-term results are equivocal

Radiographic features

Plain Radiograph ¹²

• AP

  • To calculate the coronal angle of the femoral component, an angle is made between the implant long axis and the femoral anatomic axis.

    • The acceptable range of alignment is larger for a mobile-bearing UKA compared with a relatively unforgiving fixed-bearing UKA.

    • A mobile-bearing UKA has an acceptable range of ±10 degrees for the femoral component.

  • To calculate the coronal angle of the tibial component, an angle 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.

  • The tibial component should be visualised just medial to the apex of the tibial spine, with slight 0 – 2 mm of overhang on the medial border of the tibia.

• Lateral

  • To calculate the sagittal angle of the femoral component, an angle 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.

  • To calculate the sagittal angle of the tibial component, an angle 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.

  • The femoral component should overhang the bone on the lateral proximally by 2 – 3 mm.

• Lyson Schuss view ¹³

  • This is an anteroposterior weight-bearing radiograph taken with the knee flexed at 20-30 degrees.

  • It has a 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

CT and MRI

• CT ¹⁴

  • A metal reduction CT scan can assist in answering the following clinical questions:

    • Overhang of femoral or tibial component

    • Assessment for fractures or microfractures

    • Assessing for aseptic loosening

    • Understanding UKA component rotation

    • Looking for evidence of periprosthetic infection

    • Assessing degree of polythene wear

    • Assessing for ostelysis

• MRI

  • An MRI scan has limited indications in a UKA due to metal artifact.

• Nuclear Medicine

  • A bone scan (scintigraphy) may be useful in the case of a painful UKA to look for radiotracer uptake and assess for potential aseptic loosening.

Complications ¹⁵

• Aseptic loosening

• Periprosthetic or Stress Fracture

• Periprosthetic Infection

• Polythene wear

• Polythene bearing dislocation

• Contralateral accelerated arthritis

• Impingement

• Arthrofibrosis