The native hip joint is stabilized by the bony congruence of the femoroacetabular articulation, the labrum, the surrounding hip capsule, and dynamic muscular forces across the joint. The hip capsule provides a passive restraint to extremes of hip range of motion and is reinforced by three primarily fibrous ligaments: the iliofemoral, ischiofemoral, and pubofemoral ligaments. The iliofemoral ligament has two limbs originating on the anterior inferior iliac spine that separate to form the inverted Y-shaped ligament of Bigelow as they attach on the femoral intertrochanteric line. The iliofemoral ligament primarily resists external rotation and extension, with its superolateral limb also resisting internal rotation in extension. The ischiofemoral ligament inserts on the ischium and attaches to the posterior intertrochanteric ridge; it functions primarily to reinforce the capsule during internal rotation during both hip extension and flexion. The pubofemoral ligament inserts on the superior pubic ramus and convalesces in its attachment alongside the medial iliofemoral and inferior ischiofemoral ligaments, inferomedially along the proximal femur. The pubofemoral ligament functions to reinforce the inferior hip capsule and restrict hip abduction and external rotation during hip extension. In addition to the longitudinal fibers of the aforementioned three ligaments, the circumferential fibers of the zona orbicularis around the femoral neck are also important static stabilizers of the femoral head within the acetabulum.

The study of cadaveric hip joint specimens has demonstrated the anterior capsule and iliofemoral ligament to be larger in cross-sectional area and generally less compliant than the posterior capsule. When examining differences within regions of the capsule, the highest concentration of mechanoreceptors (Ruffini, Pacinian, Golgi-like corpuscles, and free nerve endings), which are critical to proprioception and joint stability, were found in the superior region of the anterior hip capsule.¹ Capsular ligaments may adapt with progressive hip arthritis and deformity, and, in general, progressive hip arthritis leads to increased cross-sectional thickness and less compliant hip capsular ligaments.² In addition, the total number and density of mechanoreceptors were significantly decreased in arthritic hips.³

When appropriately functioning, the hip capsule may also decrease instability, bony impingement, edge loading, and wear after THA. Biomechanical cadaveric studies in which a femoral head, neck, and cup were inserted through the cotyloid fossa to preserve the integrity of the hip capsule demonstrated that adjusting THA component size and length had differential effects on the anterior and posterior capsule. The smaller head sizes used in THA, in comparison with native femoral heads, reduced the tension on the hip capsule and thus limited capsular restraint to rotation about the hip. In contrast, the authors found that hip resurfacing arthroplasty and THA with a dual-mobility implant, both incorporating larger-diameter femoral head components, showed improved capsular tension compared with THA with standard femoral head sizes of 32 or 36 mm.⁴ Specifically, the tension of the anterior iliofemoral ligament was less affected than the posterior ischiofemoral ligament by smaller femoral head sizes. The same investigation also showed that short femoral neck components may lead to capsular laxity and thus hypermobility of the hip, whereas longer femoral
neck lengths can overtighten the anterior hip capsule, thereby restricting external rotation.¹ Further research is required to better understand this interaction between hip biomechanics after arthroplasty and the integrity of the hip capsule and its role in postoperative hip stability.

IFFs are most often classified by the Vancouver system. Like the Vancouver periprosthetic fracture classification, the intraoperative Vancouver classification system also incorporates fracture location, pattern, and stability to guide management.⁹ This system involves three types of fractures: A, corresponding proximal metaphyseal fractures; B, corresponding to diaphyseal fractures; and C, corresponding to fractures distal to the stem tip. Each type has three subtypes categorized by stability, ranging from cortical perforations to nondisplaced fractures to displaced unstable fracture patterns. However, the most important step in the management of IFFs is their identification and stabilization at the time of surgery. This may involve the use of cerclage cables or other internal fixation constructs and possibly conversion to a longer stem to bypass the fracture site.

Multiple authors have reported rates of IFFs with the DAA ranging from 1% to 5.7%, with most reports between 1.3% and 2.5%.¹⁰ Matta et al¹¹ reviewed nearly 500 DAA THAs on a traction table and reported 7 (1.4%) IFFs along with 3 (0.6%) nondisplaced ankle fractures. Of these seven fractures, four were of the medial calcar during femoral broaching and three of the greater trochanter during bone hook elevation of the femur. The three ankle fractures occurred during external rotation of the limb through the table during hip dislocation and were all treated nonoperatively. Jewett and Collis¹² reported 19 (2.3%) intraoperative trochanteric fractures in a series of 800 patients undergoing the DAA on a traction table, noting most fractures occurred during femoral elevation with a bone hook. Nakata et al¹³ reported a lower IFF rate of 1.0%, which included one greater trochanteric and one calcar fracture in 195 hips. Woolson et al¹⁴ noted a higher intraoperative fracture rate of 5.7% during early transition to the DAA from a posterior approach.

A closer look at these data reveals that most IFFs in these series occurred during the early part of the series, indicating a potentially significant learning curve and a decreased fracture incidence with more surgeon experience. Of the 19 intraoperative fractures reported by Jewett and Collis,¹² 15 occurred in the first quarter of their series, and all 19 occurred in the first half of their
series of 800 patients. Similarly, Woolson et al¹⁴ reported that, although their initial complication rate was high, complications were significantly less as the authors gained more experience with the DAA.

Other reports concur with these trends; Masonis et al¹⁵ noted an increased risk of calcar fracture in the early series of a single surgeon, with three fractures in the first 62 cases (4.8%) and no subsequent fractures in the following 238 cases. Additional studies have supported these findings that most intraoperative fractures occur in the first 200 surgical cases when transitioning to the DAA.¹⁶ However, it is not inevitable that there will be a higher complication rate when transitioning to the DAA. Ponzio et al¹⁷ described the results of a single surgeon transitioning to the DAA from the posterior approach without the use of a traction table and only one femoral fracture in 289 direct anterior hips.

Although there appears to be an increased risk of IFF during the early transition to the DAA approach in some cases, there was no difference in fracture risk between those using a specialized traction table and those using a standard operating table. Moreover, despite a potential learning curve, a recent meta-analysis of the current literature by Higgins et al¹⁸ showed no significant increased risk of IFFs between the DAA and other THA approaches but noted a need for more high-quality evidence to incorporate into this type of analysis.

IFFs most frequently occur during femoral neck osteotomy, preparation of the femur, or implantation of the prosthesis. There are several important principles that may reduce the risk of IFFs during the DAA. These include preoperative templating and appropriate implant selection, adequate exposure, close attention to soft tissue tension, awareness of bony morphology and anatomic variants, and understanding the anatomy and personality of the broach and stem system used.

Greater trochanteric fractures are generally caused during femoral neck osteotomy, femoral elevation, or broach insertion or removal. The greater trochanter is at risk of iatrogenic injury when the femur is externally rotated and the greater trochanter is located posteriorly during the femoral neck osteotomy. In this position, an osteotomy directed in the anterior-posterior direction may inadvertently injure the trochanter. Thus, it is the authors’ preference to perform the femoral neck osteotomy in a neutral leg position and angle the osteotomy slightly medially to avoid iatrogenic injury to the greater trochanter. Preoperative imaging should also be examined with special attention to the morphology of the proximal greater trochanter in relation to the femoral canal.

In certain cases, the trochanter may overhang, limiting sufficient clearance to allow instrumentation of the femoral canal without injury to the trochanter. In these cases, a burr may be used to contour the trochanter to facilitate femoral broaching and stem insertion. Several studies have also noted that the use of modified instruments may aid in femoral broaching in the DAA. These include broach and stem designs incorporating single or dual offset handles and shorter broach-only stems with less prominent lateral shoulders.¹⁹ It should be noted that, although excellent results have been reported with the use of curved and offset broach handles, biomechanical studies have shown that, when compared with traditional straight handles, curved broach handles increase the moment-to-force ratio by 163% to 235%, which could theoretically increase the risk of intraoperative femoral fracture.²⁰ A key to avoiding such complications is surgeon familiarity and experience with the instrumentation selected for use.