Poster 187: Describing the Contribution of the Vacuum Effect and Hip Capsule in Restraint to Axial Traction in Patients Undergoing Hip Arthroscopy
Authors: Featherall Joseph , Metz Allan , Khalil Ameen , Rosenthal Reece , Aoki Stephen 
Published: August 2024

Objectives: In recent investigations, there has been increased focus on hip laxity and hip capsular instability, particularly in the setting of hip arthroscopy. The capsule and its internal negative pressure effects have been shown to be major contributors to hip joint stability, however, the relative contributory role of these elements remains unknown. Intraoperatively, improved understanding of hip capsule biomechanics is imperative for optimization of arthroscopic technique to access the central compartment with minimal soft tissue trauma. In turn, this can help reduce the risk of hip instability and maximize patient outcomes. The purpose of this study was to define the stiffness coefficient of the hip capsule and vacuum phenomenon in response to hip axial traction. Our hypothesis was that the vacuum phenomenon would have a greater relative contribution to axial distraction stiffness at mid-range distraction, and that the hip capsule stiffness coefficient would be greater at terminal distraction. Methods: A review of a prospectively collected database of patients undergoing hip arthroscopy was conducted from 1 July 2021 to 31 June 2022. Patients in this database underwent hip arthroscopy for the treatment of femoroacetabular impingement syndrome (FAIS). Inclusion criteria were 1) completion of the study traction protocol and 2) completion of hip joint venting prior to capsulotomy. Exclusion criteria were 1) previous hip surgery and 2) lack of hip distraction at 50 pounds of axial traction. The study traction protocol involved applying axial traction prior to instrumentation (Native State) of the hip using a post-free distraction system. Hips were then vented using a spinal needle and the traction protocol was repeated (Vented State). Traction intervals were 0, 50, and 100 lbs of force. Fluoroscopic images were taken at each traction interval and standardized to preoperative radiographs, which allowed for a calculation of the lateral joint space width at each interval. Overall distraction distance was calculated as the difference between 0 lbs traction and 50 and 100 lbs traction. The stiffness coefficient was calculated as k = traction force interval/distraction distance in both the Native and Vented States. Comparisons using paired-samples t-tests were conducted between the stiffness coefficients of the Native State and Vented State at 50 lbs and at 100 lbs. A p-value of <0.05 was utilized to indicate statistical significance. Results: Twenty-one patients were identified who met inclusion criteria. Mean age was 34.4 ± 13.4 years with 100% female and a mean BMI of 24.2 ± 4.8 points. Mean stiffness coefficient at 50 lbs in the Native State (10.9 ± 3.3) was significantly higher than the Vented State (4.9 ± 0.7, p < 0.001), with similar findings at 100 lbs in the Native State (10.1 ± 1.7) and Vented State (8.7 ± 1.3, p < 0.001). In the Vented State, the stiffness coefficient was significantly greater at 100 lbs (8.7 ± 1.3) than at 50 lbs (4.9 ± 0.7, p < 0.001). Conclusions: Our results provide an objective characterization of the individual contributions of the hip soft tissue restraints and the vacuum phenomenon to hip resistance to axial traction. At 50 pounds of traction, the vacuum effect contributes to ~55% of the axial distraction stiffness (resistance to axial distraction). At 100 pounds of traction force, the vacuum effect contributes only ~14% of the axial distraction stiffness. The resistance of the hip to axial distraction is of substantial clinical significance as minimization of traction force decreases intraoperative soft tissue trauma. This, however, needs to be balanced with obtaining sufficient working space within the hip joint. There is still substantial controversy regarding the biomechanics of the hip and the optimal methods for minimizing traction force related injury (e.g., post-less traction, minimization of traction time, air arthrogram, and joint venting). These data demonstrate that soft tissues have a proportionally greater effect on restraint of the of the hip at terminal distraction (~1 cm distraction), whereas the vacuum effect has a greater influence on mid-range distraction (~0.5 cm distraction). The proposed model for this phenomenon is that the negative pressure and the soft tissue tension work in parallel to restrain the hip joint. The increased influence of the soft tissue tension at end-range distraction may be due to a check rein effect of the soft tissues; once the iliofemoral ligament has been tensioned to its maximum length its stiffness and capacity for transfer of tensile force increases rapidly. In conclusion, at 50 pounds of traction, the vacuum effect contributes to ~55% of the axial distraction stiffness (resistance to axial distraction). At 100 pounds of traction force, the vacuum effect contributes only ~14% of the axial distraction stiffness. These results demonstrate that the vacuum effect is a significant restraint to axial traction applied across the hip at mid-range distraction. Additionally, the hip capsule has a relatively greater contribution to terminal distraction stiffness.