Three-Compartment Phenotype Concept of Total Knee Arthroplasty Alignment: Mismatch Between Distal Femoral, Posterior Femoral, and Tibial Joint Lines

BACKGROUND: The purpose of the study was to assess whether patients who have different coronal alignment variations (functional knee phenotypes FKP) have distinctly different rotational alignment variations to justify an extension of the FKP concept to include rotational alignment parameters. The goals of the study were to: (1) determine the frequency of bony congruence between the anterior, distal, posterior femoral, and proximal tibial joint lines by using the extended FKP concept; and (2) connect these findings to clinical practice by simulating the impact of different alignment concepts on the most common FKP. METHODS: The posterior condylar angle (PCA) and anterior trochlear angle (ATA) were measured in 265 knees without osteoarthritic (OA). The PCA measurements of 2,692 knees with OA were extracted from the database. The patients were categorized into phenotypes based on these parameters. A phenotype represents an alignment variation of either the posterior (= PCA) or anterior femoral joint line (= ATA) in the axial plane. Rotational phenotypes (i.e., combination of alignment variations of the anterior and posterior femoral joint lines) were linked with the coronal phenotypes of these patients. The effect of three alignment concepts (mechanical, restricted, and unrestricted kinematic) on the most common FKPs was assessed. RESULTS: The distribution of the five most common coronal phenotypes did not differ among rotational phenotypes. The ATA and PCA were aligned parallel in 14.3% of the non-OA population. Distal femoral joint line (femoral mechanical angle), proximal tibial joint line (tibial mechanical angle), and PCA were aligned parallel in 17.0 and 11.2% of the non-OA and OA populations, respectively. All four joint lines (femoral mechanical angle, tibial mechanical angle, PCA, and ATA) were aligned in 2.3% of the non-OA population. CONCLUSIONS: It is crucial to emphasize that preoperative assessment of a patient's anatomy should include the anterior and posterior femoral joint lines. The extended FKP concept could aid in this assessment and help identify patients who are at risk of complications due to malalignment or those who are likely to benefit from a particular alignment concept.