Registration of Proximal Tibial Centre May Need to be Selectively Lateralized to Avoid Coronal Malalignment in Digitally-Assisted Knee Arthroplasty

Abstract

Purpose: Placement of prosthetic components and limb alignment can be adversely impacted by errors in identification of registration points in digitally assisted knee arthroplasty. The purpose of this study was to trigonometrically analyze the impact on the accuracy of coronal tibial component and limb alignment of discrepancy between the registered midpoint of the proximal bony tibia vis-à-vis the radiographic medio-lateral center of the lateralized final tibial implant in certain varus-deformed knees.

Methods: We analyzed trigonometrically the angular difference $\theta$ in coronal alignment of tibial component and hip-knee-ankle angle resulting from lateral translation of the medio-lateral center of the final tibial implant, with respect to the initial registered midpoint of the proximal bony tibia. $\theta$ was calculated using the equation tan( $\theta$ ) = d/t, where d is the numerical distance between the registered point and the center of the final tibial tray position and t is the length of tibia distal to the resection.

Results: One degree of varus malpositioning of the tibial component and the limb can occur for every 6.3 mm of lateral translation of the tibial implant with respect to the midpoint of the proximal bony tibia of average reported length (372 mm). The magnitude of this error is inversely proportional to tibial length and the patient's height.

Conclusions: Coronal malalignment of the tibial component and the limb is directly proportional to the amount of lateral translation of the tibial tray with respect to the registered midpoint of the proximal bony tibia and inversely proportional to the patient's height. This may occur in cases with tibial extra-articular deformity, those undergoing reduction osteotomy, downsizing of the tray, or inadvertently during tibial preparation. The error can be avoided by initial registration of the tibial midpoint more laterally by templating, re-registering the new center of the resected tibial surface or osteotomy for extra-articular deformity.

Keywords: Computer-assisted knee arthroplasty; Malalignment; Robotic knee arthroplasty; Total knee arthroplasty.

Fig. 1

a Illustrates the trigonometric calculation of resultant error ($\theta$). FMA represents the femoral mechanical axis (dotted line). TMA is the dashed line between initial registration point (midpoint between the tibial eminences) on upper tibia and center of ankle joint. The solid line connects the center of the tibial implant and the center of the ankle joint. ‘d’ equals the distance between the initial registration point and final center of tibial implant after reduction osteotomy (RO) indicated by a double dashed line. ‘t’ is the length of tibia distal to the tibial resection plane. b Shows the postoperative antero-posterior full-length radiograph where computer navigation settings were dialed in to restore mechanical alignment for both components (i.e., 90° to their respective mechanical axes in the coronal plane) and the limb. Tibial registration was at the midpoint between the tibial eminences. The dotted line shows the femoral component to be orthogonal to the FMA. However, both MPTA and HKA show the tibial component and limb to be in varus

Fig. 2

a (Left) Depicts a full-length standing radiograph of a patient with a malunited proximal tibial fracture. C1 represents the point on the tibial articular surface corresponding to the templated center of the final tibial component; the resection plane P1 is such that the resected lateral tibia is of an appropriate thickness and is perpendicular to the line from C1 to center of the ankle. b (Right) C2 represents the proximal end of the tibial mechanical axis MA corresponding to the midpoint between the tibial eminences. P2 is the resection plane perpendicular to MA

Fig. 3

a (left): shows a stylized tibial component with stem extension that is correctly centered in the canal based on C1. b (center): shows that if the medio-lateral midpoint of the articular surface of the tibial component were to be centered on C2, the stem would impinge against the medial cortex and the tibial component would necessarily translate laterally to the position shown in Fig. 2c. c (right): shows correct lateral placement of the tibial component

Fig. 4

a (left): shows the tibial component aligned exactly parallel to the distal femoral resection line D which is orthogonal to the femoral mechanical axis FM; the tibial component positioning was templated and registered such that the tibial tray center was at C1. MPTA is 90° and HKA angle is 180°. b (right): shows the tibial component aligned exactly parallel to the distal femoral resection line; the tibia was cut exactly perpendicular to the preoperative MA, and the midpoint of the tibial component was translated lateral to point C2. The HKA angle and MPTA are in varus