Distinct knee phenotype variation: a comparative analysis of medial and lateral unicompartmental knee arthroplasty

Purpose: Lateral unicompartmental knee arthroplasty (UKA) is relatively less common than medial UKA. There has been no comparative analysis of the constitutional phenotypes of knees that underwent medial and lateral UKA. Therefore, this study aimed to compare the Coronal Plane Alignment of the Knee (CPAK) classification of knees that underwent medial and lateral UKA. Furthermore, the study analyzed whether CPAK phenotypes were maintained or altered after medial or lateral UKA.

Methods: We retrospectively analyzed consecutive patients who underwent UKA. A radiological analysis was conducted using an EOS imaging system, and demographic data of patients undergoing UKA were collected. Performed measurements included the mechanical hip-knee-ankle angle (mHKA), lateral distal femur angle (LDFA), and medial proximal tibia angle (MPTA), which were analyzed both before and after UKA. The CPAK classification was used to classify knee alignment phenotypes.

Results: A total of 310 knees of 244 patients were included in the study that underwent non-robotically assisted, fixed-bearing UKA (279 medial; 31 lateral). Preoperatively, the most common categories for knees were Type I (varus mechanical axis, 53.8%) in medial UKA and Type III (valgus alignment, 77.4%) in lateral UKA. Postoperatively, Type II (neutral mechanical axis) became the most common type for both groups, accounting for 34.1% in the medial UKA group and 25.8% in the lateral UKA group. Only 31.3% preserved their preoperative CPAK classification after the surgery (32.3% and 22.6%, respectively).

Conclusion: The CPAK classification differs significantly between knees that underwent medial and lateral UKA. While 31.3% of knees maintained their native knee phenotype, there is a tendency towards a neutrally aligned classification after surgery for both medial and lateral UKA. The CPAK classification optimizes preoperative categorization and may assist surgeons in tailoring personalized therapies to improve clinical outcomes.

Level of evidence: Level III.