Background
Some researchers have suggested that achieving good component coverage over the host bone during TKA (while avoiding implant overhang) may help achieve durable implant fixation and may be associated with better outcomes scores. However, the evidence about this is limited and contradictory. Contemporary morphometric TKA includes a wide array of components with various shapes and sizes, based on large anatomic databases and specific software that simulates bone cuts. Morphometric tibial components have shown improved bone coverage and better clinical outcomes than standardized implants, but the role of morphometric femoral components in bone coverage has not been studied precisely.
Questions/purposes
In a retrospective, controlled study that used patient matching, we asked: (1) Does the use of a contemporary morphometric component with more available sizes provide better femoral component fit and bone coverage than an earlier design with fewer sizes? (2) Are component fit and the presence of component overhang or underhang associated with different Knee Society Score (KSS) or Knee Injury and Osteoarthritis Outcome Score (KOOS) for Joint Replacement?
Methods
From 2012 to 2013, we performed 403 TKAs according to the following criteria: TKA performed for primary tricompartmental arthritis of the knee; varus, valgus, and flexion deformity less than 15°; and age between 18 and 85 years on the day of surgery. Among these 403 TKAs, 237 were performed using a morphometric implant and 166 with the earlier nonmorphometric implant. At 2 years of follow-up, 3% of patients in the morphometric group and 5% in the nonmorphometric group were lost to follow-up. Based on age, BMI, gender, and preoperative KSS and KOOS, two groups of 30 patients were matched in a 1:1 ratio from this longitudinally maintained database. Clinical outcomes were measured preoperatively and at a minimum follow-up of 2 years in both groups, using the KSS and KOOS. We evaluated postoperative CT images for each patient to analyze femoral implant rotation, bone coverage, and overhang and underhang status.
Results
The overhang status was similar between the two groups (23% had an overhang component in the morphometric knee group and 27% had an overhang component in the nonmorphometric knee group), and overhang was most frequently found in the lateral distal zone and medial anterior chamfer. Better cortical bone coverage was found in the morphometric knee group, with a thinner bone margin between the component edge and cortical border (morphometric group: 3 mm versus nonmorphometric knee group: 5 mm; p = 0.01). In general, there were few between-group differences in terms of patient-reported outcomes; of the seven metrics we analyzed, only the KSS favored the morphometric knee implant by a margin larger than the minimum clinically important difference (KSS mean difference: 21 points for the morphometric knee group; p < 0.05). Overhang of the femoral component of > 2 mm was associated with poorer KOOS, but not KSS, whereas a thinner bone margin had a beneficial impact on pain and global clinical scores (KOOS and KSS: p < 0.05).
Conclusion
The use of a morphometric femoral component design showed slightly improved bone fit and pain score according to the KSS at midterm follow-up compared with earlier implants with fewer sizes. Overhang > 2 mm was associated with worse KOOS. The tendency toward better outcomes in morphometric implants warrants longer-term evaluation before any definite conclusions about the association between bone fit and clinical results can be drawn.
Level of Evidence Level III, therapeutic study.
