Q&A: Total Knee Replacement: Alignment

Abstract

QUESTION: Following a total knee replacement (TKR) or total knee arthroplasty (TKA), a patient has valgus angulation of 10°. When rating the resulting impairment using Table 17-35, Rating Knee Replacement Results (5th ed, 549) of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fifth Edition, is it correct to make an alignment deduction of 30 points (3 points for each of the 10° of valgus)?ANSWER: The answer is not as simple as it may seem. The post-TKA knee reportedly has 10° of valgus alignment (also known as knock knee); however, the question does not state whether this measurement was made on physical examination or using a radiograph. The most accurate method for measuring lower limb alignment in the frontal (coronal) plane would be a full-length, standing anteroposterior radiograph, with the patella centered between the femoral condyles to ensure the condyles are oriented parallel to the frontal plane and image.The shorter the arms on a protractor, and the higher the examinee's body mass index (and resultant limb obesity), the greater the potential discrepancy between measurements on physical examination compared with measurements from a radiograph. Hence, many measurements of knee alignment on physical examination have only moderate to poor correlation with measurements derived radiographically.Note that Table 17-35 (5th ed, 549) does not state which measurement to use, but references Insall and colleagues1 and the Knee Society Scoring System (KSS©).2 The current KSS© Licensed User Manual states that alignment is measured by the femoral tibial axis on radiographs.2 Therefore, when rating impairment, clinicians should measure the anatomic, not mechanical, axes of the femur and tibia, and preferably on a radiograph.Table 17-35 (5th ed, 549) was written when findings from long-term follow-up of knee replacement were not yet published.Modern reviews of 1-year outcomes3 and 20-year outcomes4 have not identified coronal malalignment as a cause of chronic knee pain or the need for revision. However, most published studies consider “ideal” alignment on coronal full-length lower limb radiographs as a hip-knee-ankle angle between 3° of valgus and 3° of varus. This measurement of the mechanical axes is made by drawing a line from the center of the femoral head to the center of the knee, and then a line from the center of the knee to the center of the talus. Note that mechanical axes are used in outcome studies, rather than the anatomical axes employed in the KSS.Outcome studies use a different radiographic measurement that more accurately assesses the bio-mechanical consequences of implant alignment after TKA; however, when rating impairment using the AMA Guides, Fifth Edition, Table 17-35, one should measure the femoral shaft to tibial shaft angle (anatomic axes), as the KSS specifies, because this is the basis for Table 17-35.Assuming a femoral-tibial angle has been accurately measured on a postoperative full-length lower-limb radiograph, 10° of valgus is outside the desired range for component position. A range from 0° to 4° of either valgus or varus alignment is considered the goal, with no deduction of points in the scoring system. Ten degrees of valgus would qualify for 3 points per degree beyond 4°, or 3 points each for 5°, 6°, 7°, 8°, 9°, and 10°. This sums to an 18-point deduction, not 30.As an aside, the question does not state the degree of preoperative coronal plane deformity. Correcting alignment in knees with severe valgus deformity is challenging.In a situation where different examiners provided different post-TKA impairment ratings, the trier of fact may lend greater credibility to the examiner adhering to the KSS methodology and using measurements from radiographs rather than from physical examination of lower limb alignment.