Impingement and dislocation in total hip arthroplasty: mechanisms and consequences

Abstract

In contemporary total hip arthroplasty, instability has been a complication in approximately 2% to 5% of primary surgeries and 5% to 10% of revisions. Due to the reduction in the incidence of wear-induced osteolysis that has been achieved over the last decade, instability now stands as the single most common reason for revision surgery. Moreover, even without frank dislocation, impingement and subluxation are implicated in a set of new concerns arising with advanced bearings, associated with the relatively unforgiving nature of many of those designs. Against that backdrop, the biomechanical factors responsible for impingement, subluxation, and dislocation remain under-investigated relative to their burden of morbidity. This manuscript outlines a 15-year program of laboratory and clinical research undertaken to improve the scientific basis for understanding total hip impingement and dislocation. The broad theme has been to systematically evaluate the role of surgical factors, implant design factors, and patient factors in predisposing total hip constructs to impinge, sublux, and/or dislocate. Because this class of adverse biomechanical events had not lent itself well to study with existing approaches, it was necessary to develop (and validate) a series of new research methodologies, relying heavily on advanced finite element formulations. Specific areas of focus have included identifying the biomechanical challenges posed by dislocation-prone patient activities, quantifying design parameter effects and component surgical positioning effects for conventional metal-on-polyethylene implant constructs, and the impingement/dislocation behavior of non-conventional constructs, quantifying the stabilizing role of the hip capsule (and of surgical repairs of capsule defects), and systematically studying impingement and edge loading of hard-on-hard bearings, fracture of ceramic liners, confounding effects of patient obesity, and subluxation-mediated worsening of third body particle challenge.

Figure 1. Classic conceptual schematic (1975) of impingement/ dislocation, from work by Amstutz and colleagues

Figure 2. THA instability was investigated from the primary perspectives of Implant Dislocation -; Soft Tissues -; Impingement , - and Clinical Studies , , -

Figure 3. (a) Laboratory fixture for imposing THA dislocation (b) Comparison of experimental versus computational results

Figure 4. Finite element model of total hip dislocation

Figure 5. Meridional curvature of potential impingement surfaces. (a) Design concept. (b) FEA model of performance during an impingement/ subluxation event

Figure 6. (a) FE stress contours illustrating high hoop stress in the retaining ring, and (b) resisting moment parametric line plot, showing effects of change in interference fit

Figure 7. Fiber-direction-based FE representation of the hip capsule. (a) CT delineation of fiber direction, (b) overall capsule meshing structure, and (c) fiber direction distribution in a sub-section of the capsule mesh

Figure 8. (a) Retrograde sawing of the acetabular access portal; (b) Radiographic appearance of the intra-capsular construct; (c) Hemi pelvis specimen mounted in the servohydraulic hip simulator; (d) Computed versus experimentally measured resisting moments, versus simulation time, during a simulated sit-to-stand maneuver

Figure 9. Effect of capsule defect and repair on dislocation energy for (a) longitudinal full-length capsule incisions and (b) bony attachment-site releases. Caption: Acetabular 1/8 release ( ), 1/4 release ( ), 3/8 release ( ), and repair ( ); Femoral 1/8 release ( ), 1/4 release ( ), 3/8 release ( ), and repair ( ). Posterior position corresponds to the 180° circumferential location

Figure 10. (a) Posterior capsule incision as typically used during a posterolateral surgical approach, and two distinct capsule repair variants using a single suture (b) or six equally-spaced sutures (c). When too few sutures are used, high tensile stresses occur within the suture (white arrow). For parametric analyses of suture position and number (d), posterior capsule incisional repairs with fewer than four sutures were predicted to fail (gray band identifies range of suture failure loads). Caption: Acetabular 1/8 detachment repair ( ); Femoral 1/8 detachment repair ( ); Longitudinal incision repairs with two (‡ ), three ( ), six ( ) or nine () sutures

Figure 11. (a): FE model of HoH THA impingement, demonstrating stress concentrations at the impingement and egress sites (femoral component rendered translucent, and the posterior half of the capsule transparent, for clarity). Using a submodeling formulation (b,c), highly accurate stress resolutions at these sites are possible, at a minimum of computational cost

Figure 12. Kinetic range of motion (ROM) for 77 permutations of cup lip radius and cup inclination (a) and anteversion (b). In general, ROM was reduced for increased values of cup lip radius and decreased cup inclination and anteversion. The changes in ROM due to lip radius change were most pronounced for the more horizontally oriented cups. Contact pressure contours for the 5-mm lip radius cases are shown in the lower insets

Figure 13. (a) Three head sizes in common usage with HoH THA were studied. Five dislocation maneuvers were considered (predisposing both to anterior and posterior dislocation), for parametric variations in cup orientation straddling the THA “safe zone.” Impingement, instability, and mean peak stresses decreased with larger heads (b). Competing considerations, stability and surface stresses, were combined into a novel metric of THA performance (c), to determine optimal placement as a function of head size

Figure 14. Linear-elastic fracture mechanics (LEFM) model of ceramic liners. LEFM theory is predicated upon a pre-existing flaw. The existence of microscopic cracks due to manufacturing were presumed to constitute a source of flaws. Specialty quarter-point 2nd order fracture elements were arranged in a rosette pattern (a,b) to allow for fracture propagation calculations. During impingement/subluxation, the crack face (c) opens, with resulting high stress near the crack tip (d). (e) Stress intensity factors (K) for seven impingement-prone motion challenges. Three kinematic sequences developed KI values in excess of alumina's critical stress intensity for fracture propagation (4 MPa m^1/2)

Figure 15. (a) Deep flexion during squatting leads to near-edge loading and development of high stress at an intermediate location between the cup edge and cup pole. These stresses are passed to an XFEM submodel (b), which allowed for both fracture initiation and crack propagation to be modeled, without the requirement for effort-intensive specialized meshes

Figure 16. The obesity dislocation model (a) consisted of THA hardware and the hip capsule, plus mirrored right and left thighs (comprised of skin, adipose tissue, bulk muscle and the femur). Material properties for skin and muscle were assumed linearly elastic, with the adipose tissue treated with a hyperelastic constitutive material model. Material coefficients were obtained from literature. Using anthropometric and cadaveric data, a total of eight graded-levels of obesity were considered (b)

Figure 17. (a) During a sit-to-stand maneuver, thigh-thigh impingement induces a laterally-directed force of sufficient magnitude to cause subluxation and occasionally frank dislocation in the more obese simulations. It was observed that with the 28mm implant (b, top panel), this obesity effect on stability becomes appreciable for BMI values >40. When using a standard offset 36mm implant (b, middle panel), only minor improvement was observed, especially for horizontal cups, and the BMI threshold remained unchanged at 40. However, substantial improvement in stability was observed when using an 8mm high-offset neck (b, bottom panel), with only minor subluxation detected even for the highest BMI simulations