Custom total knee arthroplasty facilitates restoration of constitutional coronal alignment

Abstract

Purpose: To describe a strategy for coronal alignment using a computed tomography (CT) based custom total knee arthroplasty (TKA) system, and to evaluate the agreement between the planned and postoperative Hip-Knee-Ankle (HKA) angle, Femoral Mechanical Angle (FMA) and Tibial Mechanical Angle (TMA).

Methods: From a consecutive series of 918 primary TKAs, 266 (29%) knees received CT-based posterior-stabilized cemented custom TKA. In addition to a preoperative CT-scan, pre- and post-operative radiographs of weight-bearing long leg, anterior-posterior and lateral views of the knee were obtained, on which the FMA, TMA and HKA angles were measured. CT-based three-dimensional (3D) models enabled to correct for cases with bony wear by referring to the non-worn areas and to estimate the native pre-arthritic angles. The alignment technique aimed to preserve or restore constitutional alignment (CA) within predetermined limits, by defining a 'target zone' based on three criteria: 1) a ± 3° (range 87°-93°) primary tolerance for the femoral and tibial resections; 2) a ± 2° secondary tolerance for component obliquity, extending the bounds for FMA and TMA (range 85°-95°); 3) a planned HKA angle range of 175°-183°. Agreement between preoperative, planned and postoperative measurements of FMA, TMA and HKA angle were calculated using intra-class correlation coefficients (ICC).

Results: Preoperative radiograph and CT-scan measurements revealed that, respectively, 73 (28%) and 103 (40%) knees were in the 'target zone', whereas postoperative radiographs revealed that 217 (84%) TKAs were in the 'target zone'. Deviation from the planned angles were - 0.5° ± 1.8° for FMA, - 0.5° ± 1.8° for TMA, and - 1.1° ± 2.1° for HKA angle. Finally, the agreement between the planned and achieved targets, indicated by ICC, were good for FMA (0.701), fair for TMA (0.462) and fair for HKA angle (0.472).

Conclusion: Using this strategy for coronal alignment, 84% of custom TKAs were within the 'target zone' for FMA, TMA and HKA angles. These findings support the concepts of emerging personalized medicine technologies, and emphasise the importance of accurate strategies for preoperative planning, which are key to achieving satisfactory 'personalised alignment' that can further be improved by customisation of implant components.

Level of evidence: IV.

Keywords: Coronal alignment; Custom TKA; FMA; HKA; Patient-specific; TMA; Total knee arthroplasty; Total knee replacement.

Fig. 1

Flowchart of the study cohort

Fig. 2

This matrix represents FMA on the horizontal axis and TMA on the vertical axis. Varus femurs were defined as FMA < 91° and Valgus femurs as FMA > 95°; Varus tibias were defined as TMA < 85° and Valgus tibias as TMA > 89°. Overall limb alignment was considered in varus if HKA angle < 177° and in valgus if HKA angle > 183°. Finally nine morphotypes were defined based on FMA and TMA

Fig. 3

a Matrix shows the alignment possibilities offered by the Origin^® system. A primary tolerance of ± 3° for the femoral and tibial cuts, which were planned to achieve a range of mechanical angles from 87° to 93° (yellow area). A secondary tolerance of ± 2° for implant obliquity (polyethylene insert and femoral condyles), which extended the total range of mechanical angles from 85° to 95° (Orange area). b ‘Target zone’ (Yellow area) respecting the 85° to 95° range for FMA and TMA, and the 175° to 183° range for HKA angle

Fig. 4

When the preoperative FMA and TMA are within the ‘target zone’, the constitutional alignment is maintained postoperatively. When the preoperative FMA and TMA are outside the ‘target zone’, the alignment is corrected

Fig. 5

a In this patient, the global radiograph varus deformity is a combination of arthritic deformity (bone wear and laxity) and constitutional deformity (HKA angle = 168°). Constitutional alignment is outside the ‘target zone’ (FMA = 91° and TMA = 82°). The planning corrects the deformity to the ‘target zone’ (FMA = 92°, TMA = 85°, HKA angle = 177°). b In this patient, the global radiograph valgus deformity is a combination of arthritic deformity (bone wear and laxity) and constitutional deformity (HKA angle = 198°). Constitutional alignment is outside the ‘target zone’ (FMA = 97° and TMA = 91°). The planning corrects the deformity to the ‘target zone’ (FMA = 93°, TMA = 89°, HKA angle = 182°)

Fig. 6

258 knees of the series are included in these matrices with FMA and TMA measured on: a preoperative long-leg radiographs, b preoperative CT-scans, and c postoperative long-leg radiographs

Fig. 7

Examples where varus cuts have different consequences. In patient A, bone wear is minimal and most of the global varus deformity (16°) is due to its native architecture (constitutional varus). An orthogonal cut induce an asymmetric resection, in a weak zone of the lateral tibial plateau. With a varus cut there is less asymmetry and the bone cut remain in the subchondral area. In patient B, bone wear is severe and most of the global varus deformity (18°) is due to the bone loss (no constitutional varus). An orthogonal cut corrects the bone loss and do not induce asymmetric resection. A varus cut is very oblique with respect to the tibial shape and is situated below the subchondral area on the medial tibial plateau