Medial Unicompartmental Arthroplasty of the Knee

Jason M. Jennings, MD, DPT; Lindsay T. Kleeman-Forsthuber, MD; Michael P. Bolognesi, MD


J Am Acad Orthop Surg. 2019;27(5):166-176. 

In This Article

Preoperative Imaging

Radiographic Evaluation

Evaluation begins with standard radiographic views of the knee, including standing AP, lateral, merchant, and Rosenberg. The lateral view can indicate ACL insufficiency if evidence of posterior tibial erosion or posterior femoral subluxation exists. The merchant view assesses the PFJ and should not demonstrate lateral patella subluxation or lateral patella erosion.

Valgus stress views have been used to determine whether a patient's varus deformity is correctable and to assess the lateral compartment cartilage integrity.[16] The stress view is performed by flexing the knee 20° and applying a valgus force with the x-ray beam perpendicular to the knee joint (Figure 1). If the lateral joint space maintains a width of 5 mm or more and if the mechanical varus alignment can be corrected to within 3° of neutral, then the UKA can be considered.[16] Waldstein et al[16] studied the efficacy of valgus stress views in predicting lateral compartment degenerative changes and found no correlation between lateral joint space width on stress view and Outerbridge grade of the lateral cartilage intraoperatively. They found gross variations in measurements based on age and gender with males and younger patients having a markedly wider joint space. Overall, stress radiographs have not demonstrated a clear benefit in determining the severity of lateral compartment arthritis but are still useful for determining whether the mechanical alignment is correctable.

Figure 1.

Radiographs showing an example of standing AP (A) and valgus stress (B) of the knee in the preoperative work-up of the patient being considered for unicompartmental knee arthroplasty. Stress radiograph is performed with the knee in slight flexion with x-ray beam perpendicular to the knee joint. A valgus force is placed on the knee joint to compress the lateral compartment soft tissue. Narrowing greater than 5 mm of the lateral joint space with valgus stress is considered indicative of lateral compartment degenerative changes.16

Cross-sectional Imaging Evaluation

MRI can be a poor indicator of knee pathology severity and even falsely overestimate the extent of cartilage damage or ACL deficiency.[17] Hurst et al[17] studied outcomes of UKA in patients with abnormal and normal preoperative MRIs and found no difference in pain or functional scores between groups with only one failure in the abnormal MRI group compared with four failures in the normal/no MRI group. Other studies have demonstrated similar findings with MRI providing over exaggerated severity of knee pathology and not being associated with increased risk of failure after UKA.[18] For patients with suspected spontaneous osteonecrosis of the knee, MRI can be used to confirm the diagnosis and assess the extent of the disease.

CT is primarily used for patients undergoing robotic-assisted UKA. A three-dimensional model of the patient's knee is generated to guide implant positioning and soft-tissue balancing.