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Abstract

INTRODUCTION: Increasing the amount of posterior tibial slope in Total Knee Arthroplasty (TKA) was suggested as a manner to release a tight flexion gap. However, an excessive tibial slope could jeopardize the knee stability in flexion. The effects of varying tibial slope on the amount of laxity of the knee joint remain unclear. The aim of this study was to investigate the effect of tibial slope on the anterior-posterior (AP) and varus-valgus (VV) envelopes of laxity in TKA. We hypothesized that the effect of slope on laxity will also depend on the surgical technique used; therefore we simulated laxity tests with tibial slope added using both an anterior tibial cortex referencing (ACR) technique and a center of tibial plateau referencing (CPR) technique. METHODS: A previously validated musculoskeletal model of a 86-year-old male subject, having a cruciate-retaining TKA prosthesis, was configured to simulate AP and VV laxity tests [1]. First the model was simulated without any external loads applied, with the knee spanning a 0-90 degrees range of motion (ROM). Subsequently, anterior and posterior loads of 70 N were applied alternately to the proximal tibia, and the resulting AP tibial displacement recorded throughout the knee ROM. Similarly, varus and valgus loads of 15 Nm were applied alternately to the tibia and the resulting knee VV rotation recorded. The same loading conditions were replicated in four additional simulation cases with -3, +3, +6, +9 degrees of posterior tibial slope, added using the ACR technique (Fig. 1a), and four more cases using the CPR technique (Fig. 1b). For all slope cases laxity curves were calculated as the displacement or rotation recorded in the loaded cases minus the one recorded in the unloaded case. The laxity calculated with no additional slope (0°) constituted the baseline. RESULTS: In the baseline, anterior and posterior laxities were both 2 mm, in extension, and 8 and 6 mm, respectively, at 90° flexion, but they reached a maximum at 60° flexion (10 and 7 mm, respectively). Varus and valgus laxities were both 1°, in extension, and increased to 3° and 4°, respectively, at larger knee flexion angles. The effect of tibial slope on AP and VV laxities was markedly different, depending on the referencing technique. A more posterior tibial slope with the ACR technique caused a gradual increase in the anterior, varus and valgus laxities, at all knee flexion angles and interestingly also in extension. The anterior laxity was maximal (23 mm) at 60° of knee flexion in the +9° ACR case. The posterior laxity increased with more slope, in extension, but was largest with 3° and 6° of slope, around mid-flexion. Conversely, a variation of tibial slope obtained by using the CPR technique hardly affected the AP and VV laxities. DISCUSSION: The knee stability is compromised if additional slope is attained using the ACR technique: both anterior-posterior and varus-valgus laxity increased with increased tibial slope in flexion (as intended) but also in extension. In contrast, the CPR technique preserves the translational and rotational laxity envelopes of the knee, throughout the ROM. More posterior tibial slope intra-operatively should be attained using the CPR technique, in order to preserve the knee stability. SIGNIFICANCE: This study contributes to a deeper understanding of the effect of tibial slope in TKA under simulated laxity tests, and used a highly controlled and parameterized study design. Orthopedic surgeons can directly benefit from this study, as the results can be readily translated into indications for the clinical practice. The presented tool can also be very useful for educational/medical training purposes. REFERENCES: [1] Marra MA, Vanheule V, Fluit R, et al. A Subject-Specific Musculoskeletal Modeling Framework to Predict In Vivo Mechanics of Total Knee Arthroplasty. ASME. J Biomech Eng. 2015;137(2):020904-020904-12 ACKNOWLEDGEMENTS: The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 323091 awarded to N. Verdonschot.
The Effect of Posterior Tibial Slope on Simulated Laxity Tests in Cruciate-Retaining TKA
Marco A. Marra1, Marta Strzelczak1, Petra J.C. Heesterbeek2, Sebastiaan van de Groes1, Dennis Janssen1, Bart F.J.M. Koopman3, Ate B. Wymenga2,
Nico Verdonschot1,3
1Radboudumc, Nijmegen, The Netherlands, 2Sint Maartenskliniek, Nijmegen, The Netherlands, 3University of Twente, Enschede, The Netherlands,
Disclosures: M. Marra: None. M. Strzelczak: None. P. Heesterbeek: None. S. van de Groes: None. D. Janssen: None. B. Koopman: None. A. Wymenga:
None. N. Verdonschot: None.
INTRODUCTION: Increasing the amount of posterior tibial slope in Total Knee Arthroplasty (TKA) was suggested as a manner to release a tight flexion
gap. However, an excessive tibial slope could jeopardize the knee stability in flexion. The effects of varying tibial slope on the amount of laxity of the knee
joint remain unclear. The aim of this study was to investigate the effect of tibial slope on the anterior-posterior (AP) and varus-valgus (VV) envelopes of
laxity in TKA. We hypothesized that the effect of slope on laxity will also depend on the surgical technique used; therefore we simulated laxity tests with
tibial slope added using both an anterior tibial cortex referencing (ACR) technique and a center of tibial plateau referencing (CPR) technique.
METHODS: A previously validated musculoskeletal model of a 86-year-old male subject, having a cruciate-retaining TKA prosthesis, was configured to
simulate AP and VV laxity tests [1]. First the model was simulated without any external loads applied, with the knee spanning a 0-90 degrees range of
motion (ROM). Subsequently, anterior and posterior loads of 70 N were applied alternately to the proximal tibia, and the resulting AP tibial displacement
recorded throughout the knee ROM. Similarly, varus and valgus loads of 15 Nm were applied alternately to the tibia and the resulting knee VV rotation
recorded. The same loading conditions were replicated in four additional simulation cases with -3, +3, +6, +9 degrees of posterior tibial slope, added using
the ACR technique (Fig. 1a), and four more cases using the CPR technique (Fig. 1b). For all slope cases laxity curves were calculated as the displacement or
rotation recorded in the loaded cases minus the one recorded in the unloaded case. The laxity calculated with no additional slope (0°) constituted the
baseline.
RESULTS: In the baseline, anterior and posterior laxities were both 2 mm, in extension, and 8 and 6 mm, respectively, at 90° flexion, but they reached a
maximum at 60° flexion (10 and 7 mm, respectively). Varus and valgus laxities were both 1°, in extension, and increased to 3° and 4°, respectively, at larger
knee flexion angles. The effect of tibial slope on AP and VV laxities was markedly different, depending on the referencing technique. A more posterior tibial
slope with the ACR technique caused a gradual increase in the anterior, varus and valgus laxities, at all knee flexion angles and interestingly also in
extension. The anterior laxity was maximal (23 mm) at 60° of knee flexion in the +9° ACR case. The posterior laxity increased with more slope, in
extension, but was largest with 3° and 6° of slope, around mid-flexion. Conversely, a variation of tibial slope obtained by using the CPR technique hardly
affected the AP and VV laxities.
DISCUSSION: The knee stability is compromised if additional slope is attained using the ACR technique: both anterior-posterior and varus-valgus laxity
increased with increased tibial slope in flexion (as intended) but also in extension. In contrast, the CPR technique preserves the translational and rotational
laxity envelopes of the knee, throughout the ROM. More posterior tibial slope intra-operatively should be attained using the CPR technique, in order to
preserve the knee stability.
SIGNIFICANCE: This study contributes to a deeper understanding of the effect of tibial slope in TKA under simulated laxity tests, and used a highly
controlled and parameterized study design. Orthopedic surgeons can directly benefit from this study, as the results can be readily translated into indications
for the clinical practice. The presented tool can also be very useful for educational/medical training purposes.
REFERENCES: [1] Marra MA, Vanheule V, Fluit R, et al. A Subject-Specific Musculoskeletal Modeling Framework to Predict In Vivo Mechanics of
Total Knee Arthroplasty. ASME. J Biomech Eng. 2015;137(2):020904-020904-12
ACKNOWLEDGEMENTS: The research leading to these results has received funding from the European Research Council under the European Union's
Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 323091 awarded to N. Verdonschot.
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