High inter- and intraindividual differences in medial and lateral posterior tibial slope are not reproduced accurately by conventional TKA alignment techniques

Abstract

PurposeThe purpose of this study was to describe the medial and lateral posterior tibial slope (MPTS and LPTS) on 3D‐CT in a Caucasian population without osteoarthritis. It was hypothesised that standard TKA alignment techniques would not reproduce the anatomy in a high percentage of native knees. MethodsCT scans of 301 knees [male:female = 192:109; mean age 30.1 (± 6.1)] were analysed retrospectively. Tibial slope was measured medially and laterally in relation to the mechanical axis of the tibia. The proportion of MPTS and LPTS was calculated, corresponding to the “standard PTS” of 3°–7°. The proportion of knees accurately reproduced with the recommended PTS of 0°–3° for PS and 5°–7° for CR TKA were evaluated. ResultsInterindividual mean values of MPTS and LPTS did not differ significantly (mean (range); MPTS: 7.2° ( – 1.0°–19.0°) vs. LPTS: 7.2° ( − 2.4°–17.8°), n.s.). The mean absolute intraindividual difference was 2.9° (0.0°–10.8°). In 40.5% the intraindividual difference between MPTS and LPTS was > 3°. When the standard slope of 3°–7° medial and lateral was considered, only 15% of the knees were covered. The tibial cut for a PS TKA or a CR TKA changes the combined PTS (MPTS + LPTS) in 99.3% and 95.3% of cases, respectively. ConclusionA high interindividual range of MPTS and LPTS as well as considerable intraindividual differences were shown. When implementing the recommended slope values for PS and CR prostheses, changes in native slope must be accepted. Further research is needed to evaluate the impact of altering a patient’s native slope on the clinical outcome. Level of evidenceIV.

Restoring the Patient’s Pre-Arthritic Posterior Slope Is the Correct Target for Maximizing Internal Tibial Rotation When Implanting a PCL Retaining TKA with Calipered Kinematic Alignment

“…The present study suggests that the correct target for setting the tibial component with a calipered KA TKA and PCL retention is the patient's individual pre-arthritic slope within a tolerance of ±2 • because the 15-19 • range of internal tibial rotation was comparable to the native knee and more significant deviations increase the risk of tibial component failure [4,6,11,17]. The fixed 3-7 • slope range recommended for MA does not account for the 20 • inter-individual range of the pre-arthritic posterior slope and should not be used with a calipered KA as only 33% (7 of 21) of the tibial components in the present study fit within this range [8][9][10]17]. MA surgeons commonly use techniques such as increasing the posterior slope and PCL recession and release to increase knee flexion; however, they cause a loss of internal tibial rotation and risk tibial component failure [3,5].…”

“…The correct target for setting the posterior slope with PCL retention is debatable and depends on the alignment method. A target recommended for mechanical alignment (MA) is 3-7 • of the posterior slope [8]. However, a 3-7 • range does not account for the 20 • inter-individual range of the native posterior slope, and its use changes PCL tension in most knees [8][9][10].…”

“…A target recommended for mechanical alignment (MA) is 3-7 • of the posterior slope [8]. However, a 3-7 • range does not account for the 20 • inter-individual range of the native posterior slope, and its use changes PCL tension in most knees [8][9][10]. Because the PCL tension in flexion drives tibial rotation, setting the tibial component to an incorrect slope might cause a loss of internal tibial rotation, thereby compromising the coupled reduction in the Q-angle throughout knee flexion, adversely affecting the retinacular ligaments' tension and patellofemoral tracking [11][12][13][14].…”

“…Intraoperatively, the tibial resection slope is set parallel to the medial joint line's posterior slope by adjusting the plane of an angel wing inserted in the tibial guide (Figure 1). Since the posterior slope on a lateral radiograph is minimally affected by arthrosis, as long as the medial and lateral tibial plateaus closely superimpose, the difference between the pre and postoperative slope can determine the angel wing technique's accuracy [7][8][9]. There is a presumption that when a calipered KA retains the PCL and uses components that closely match the surface conformity of the native knee, the coupled internal rotation during passive flexion is also restored.…”

Comparison of the Tibial Posterior Slope Angle Between the Tibial Mechanical Axis and Various Diaphyseal Tibial Axes After Total Knee Arthroplasty

“…Second, the conditions of both condyles of the tibia in non-TKA knees, such as native, ACL-injured, and OA knees, were different from those of TKA knees. For instance, non-TKA knees have an asymmetric size [ 26 , 27 ] and different posterior inclinations [ [27] , [28] , [29] ] between the medial and lateral condyles although TKA knees in this study were symmetric and had similar inclination between them. That indicates that measurement errors are likely to occur in determining the midpoint of the width of condyles of different sizes and drawing the tangent line of different inclinations between condyles in previous non-TKA knees.…”