Distal Femoral Valgus Resection Angle in Conventional Total Knee Arthroplasty - a CT Scanogram Study.

) 180( COPYRIGHT 2021 © BY THE ARCHIVES OF BONE AND JOINT SURGERY RESEARCH ARTICLE Distal Femoral Valgus Resection Angle in Conventional Total Knee Arthroplasty - a CT Scanogram Study Praveen L. Basanagoudar, MS, DNB, MRCS; Bhava R.J. Satishkumar, MS, DNB; Kirubakaran Pattabiraman, MS, DNB, MRCS; Dinesh Kamath, MS; Ranganadham AV, MS Research performed at Sagar Hospital, DSI institutions, India Received: 2 October 2022 Accepted: 12 November 2022 Abstract Objectives: In conventional total knee arthroplasty (TKA), the distal femur valgus resection angle (DFVA) is decided either by measuring the specific resection angle for each patient on preoperative anteroposterior hip-knee-ankle (HKA) weight-bearing radiograph or using a fixed resection angle of five to seven degrees, when such facilities are not available. This study aims to measure the DVFA in TKA patients using preoperative HKA non-weight-bearing computerized tomography (CT) scanogram scout films and determine its relation with preoperative coronal plane lower-limb deformities. Methods: In this retrospective radiological study, various measurements were performed on bilateral, preoperative hip-knee-ankle CT scanograms of 73 knee osteoarthritis patients who had presented for total knee replacement surgery using a standard protocol. The angle between the femoral anatomical axis and femoral mechanical axis was measured as the femoral mechanical anatomical angle (FMAA), which corresponds to the surgical DFVA. The angle between the femoral and tibial mechanical axes was measured as mechanical femorotibial angle (MFTA). The correlation between FMAA and MFTA was studied. Results: The mean FMAA for the study group was 6.45° (range 3° to 11°, SD 1.17°). The MFTA for the study group ranged from 24° varus to 14° valgus. The alignment was valgus in 14.4% (n=21), varus in 84.2% (n=123), and “0 degrees” in 1.3% (n=2). With valgus coronal alignment taken as positive and varus as negative, the Pearson's correlation coefficient for MFTA with FMAA was r = −0.5183 (p<0.001), indicating that valgus knees tended to have a smaller FMA angle and varus knees tended to have a larger FMA angle. Conclusion: In the non-availability of individualized measurements, in primary TKA, we recommend setting DFVA as five degrees for valgus deformities, six degrees for mild/moderate varus deformities (MFTA <15°) and seven degrees for severe varus deformities (MFTA > 15°). Level of evidence: III Keywords: CT study, Distal femur, Knee arthroplasty, Knee osteoarthritis, Resection angle, Valgus cut angle Introduction T otal knee arthroplasty (TKA) is one of the most successful orthopedic procedures performed in patients with end-stage knee osteoarthritis for alleviating the symptoms and correcting deformity, with satisfactory long-term survival rates. Common causes for failure of TKA include aseptic loosening, malalignment, instability, and infection.1 Proper limb alignment and implant component positioning are critical to the longterm survivorship of TKA.2 Incorrect mechanical alignment is related to early implant wear, loosening, and prosthesis Corresponding Author: Praveen L. Basanagoudar, Sagar Hospitals, India Email: instability.1, 3Proper mechanical alignment of the lower limb is achieved by appropriate bone cuts and soft tissue balancing. To achieve mechanical alignment, the distal femur and proximal tibia are cut at right angles to their mechanical axes.4,5 Postoperative alignment within a range of +/- 3 degrees )coronal “safe zone”) from the mechanical axis is aimed for, which is associated with better outcomes.2,6,7,8 However, some clinical studies could not correlate better alignment with better implant survivorship on long-term follow-up.4,5,9,10 Despite the THE ONLINE VERSION OF THIS ARTICLE ABJS.MUMS.AC.IR Arch Bone Jt Surg. 2023; 11(3): 180-187 Doi: 10.22038/ABJS.2022.67615.3216 http://abjs.mums.ac.ir (181) THE ARCHIVES OF BONE AND JOINT SURGERY. ABJS.MUMS.AC.IR VOLUME1 1. NUMBER 3. MARCH 2023 controversy, one of the goals of conventional TKA is to aim for coronal “safe zone” postoperative lower limb alignment by reducing the outliers for postoperative mechanical axis correction. This is achieved in TKA by time-tested conventional methods and, more recently, by assistive devices like computer navigation, patient-specific instrumentation, or robotics.11-13 In conventional TKA using the mechanical alignment method, an intramedullary (IM) guide is used for performing the distal femoral cut. To achieve a distal femur cut perpendicular to the femoral mechanical axis, the distal femoral cutting guide over the IM guide is set to five or six degrees of valgus. This method assumes that the femoral anatomical axis is five or six degrees valgus in relation to the femoral mechanical axis, which may be appropriate for the majority of the patients.14,15 Using a fixed valgus angle may result in either mal-alignment of the femoral component or outliers in the correction of coronal plane deformity outside the acceptable range, in 10% to as high as 30 % of patients undergoing TKA.16-18 Studies on healthy non-arthritic populations have found the angle between the femoral mechanical axis and femoral anatomical axis (FMAA) to be in the range of 5.1 to 5.8 degrees.19,20 Ideally, pre-operative weight-bearing anteroposterior long leg Hip-Knee-Ankle radiographs (HKA-WBR) need to be done in each patient to determine FMAA, which can be used as distal femur valgus resection angle (DFVA) or valgus cut angle (VCA) for that specific patient. However, considerable variations in FMAA values Distal femoral valgus cut in total knee arthroplasty in the knee osteoarthritis population have been noted. A review of the recent literature shows evidence in favor of patient-specific DFVA with a significant decrease in the percentage of postoperative outliers for mechanical axis correction [Table 1]. In the Indian setup, many hospitals do not have facilities for HKA-WBR to determine preoperative patient-specific FMAA. Many surgeons rely on fixed FMAA/ DFVA/VCA values of either five or six degrees for all TKAs. While weight-bearing lower limb radiograph is considered the gold standard, Computed tomography (CT) scanograms are also used to calculate FMAA. CT scanograms are quite popular, given lower radiation exposure, speed of data acquisition, and convenient supine positioning.11,21 CT values were reported to be more accurate and reproducible than the plain radiographs.21, 22 Previous studies in the Indian population using HKA-WBR have found wide variation in FMAA values ranging as low as 1.4 to as high as 11.4 degrees, the mean varying from 5.9 to 6.9 degrees.17, 23 Our study aims to assess the natural distribution of the FMAA in an Indian osteoarthritic population presenting for TKA, using preoperative supine CT scanograms and to find any correlation between FMAA and the preoperative lower limb coronal plane deformity as measured by mechanical femorotibial angle (MF (...truncated)