In physiologically young patients with displaced femoral neck fractures, surgical treatment is aimed at achieving fracture union while preserving native hip anatomy and biomechanics. The unique intracapsular fracture environment of these fractures impedes typical fracture healing mechanisms because of the presence of synovial fluid (impeding clot formation necessary in secondary bone healing) and lack of cambium periosteal layer (which provides proliferative and osteogenic cells).1,2 Fractures in this area therefore rely on primary bone healing. Stable fixation that resists shear and rotational forces and fracture compression are paramount in facilitating healing.3 Achieving anatomic reduction is critical to decrease the rates of fracture displacement, nonunion, and osteonecrosis of the femoral head.4,5
Studies have demonstrated the importance of the superior and inferior retinacular arteries in perfusion of the superior weight-bearing and inferior aspects of the femoral head, respectively.6 The presumed cause of osteonecrosis is multifactorial, including decreased arterial inflow and venous outflow from the femoral head, disruption of terminal vessels from initial trauma, and vascular tamponade as a result of an intracapsular fracture hematoma.6 It is postulated that an important source of femoral head revascularization after a fracture is ingrowth across the fracture line as it heals, and this can be easily disrupted by micromotion at the fracture site.7 Despite attempts at anatomic fixation and capsular decompression, osteonecrosis of the femoral head that has been reported to occur in 10%–30% of femoral neck fractures is common regardless of the fixation method.8–11
In addition, the biomechanics of the native hip joint are unfavorable for healing of vertically oriented fractures. Fixation constructs must withstand the normal forces about the hip including compressive, tensile, and torsional strains until fracture healing has occurred. Fractures with high Pauwels angles12,13 or posterior femoral neck comminution14,15 present a more biomechanically disadvantageous environment for fracture healing. Whether resulting from inadequate perfusion or biomechanical instability, or both, nonunion rates of 7%–33% have been reported.8,9,12,16
Conventional fixation methods for osteosynthesis of femoral neck fractures include multiple cancellous screws, fixed-angle dynamic implants, and fixed-angle length-stable constructs. Despite several biomechanical and clinical studies evaluating various surgical options, the optimal fixation construct to allow healing and prevent nonunion of displaced femoral neck fractures is not known. This article will describe the technique and rationale behind novel alternative treatment options for these challenging fractures.
Fixed-Angle Length-Stable Options
Proximal femoral locking plates (PFLPs) were developed as fixed-angle length-stable options in an effort to decrease femoral neck shortening after fixation of femoral neck fractures (which is seen in 30% of femoral neck fractures treated with multiple cannulated screws (MCSs) and is associated with inferior patient-reported outcomes17,18). Early biomechanical results of PFLPs were promising, showing increased axial stiffness, increased resistance to shear forces, decreased toggling, and decreased micromotion of PFLPs compared with other traditional fixation methods1,19–22; however, clinical results have been disappointing.23 Overall, biomechanical data cannot fully represent physiologic loading that occurs in clinical use, and comparisons between biomechanical studies are difficult because of varying fracture and loading models, as well as nuances between similar constructs.
Although PFLPs showed good biomechanical results compared with other fixed-angle constructs, the only clinical study of their use in acute femoral neck fractures revealed unacceptable rates of catastrophic failure.23 In this retrospective study with a minimum of 1-year follow-up, the implant used included a side plate with 5 locking screws directed from the plate toward the femoral head (two 7.3-mm and two 5.0-mm screws at converging/diverging angles along the neck and a single 4.5-mm screw along the femoral calcar) and a single locking or cortical screw in the shaft. Open reduction was performed in 16 of 18 patients, and 17 achieved anatomic reduction (as deemed by <2 mm of fracture site step-off or <5 degrees of side-to-side variation in varus/valgus alignment on immediate postoperative imaging). Catastrophic failure occurred in 36.7% (7 of 18 cases) in the overall cohort and 63.6% (7 of 11 cases) in the displaced femoral neck fracture subgroup. None of the Garden I and II fractures experienced mechanical failure before healing. Failure modes seen in these catastrophic failures included broken screws with varus displacement (n = 5), femoral head collapse with intra-articular penetration (n = 1), and dissociation of the distal screw from the shaft (n = 1). Union rates were 61.1% (11 of 18). When compared with historical controls of patients treated with open reduction, and fixed-length construct of MCSs at the same institution, the clinical and radiographic results of PFLPs were far inferior despite no difference in age between the cohorts.23 The authors theorized that the rigidity of the construct prevented controlled collapse to accommodate for fracture site resorption, preventing bone-to-bone compression and placing the mechanical stresses on the implant itself.
Although the PFLP construct in theory would allow fixed-angle fixation while maintaining an anatomic reduction with length-stable locking screws, its use sharply declined after reports of clinical failure. Recent data have suggested an increased risk of decreased mechanical stability and construct failure with off-axis (>5 degrees) placement of locking screws and fracture malreduction.24,25 Clinical failures occur at the bone–screw interface or fatigue failure of the plate and are theorized to be due to the implant stiffness that prevents compression and micromotion necessary for primary bone healing.23,26 Previous studies have demonstrated the deleterious effects of excessive rigidity on fracture healing.26–28 Furthermore, the strain theory would indicate that rigid fixation of a fracture with a microscopic gap present is more deleterious for healing than micromotion at an anatomically reduced fracture.29 Therefore, the failure of a properly applied, seemingly biomechanically superior, locking plate construct is likely two-fold: intrinsic construct rigidity causing excessive fracture gap strain and the unique intracapsular environment, which lacks the cellular capacity to form a callus in an effort to diminish the high local strain.
Finally, blade plates are a fixed-angle length-stable fixation option rarely used as primary fixation in femoral neck fractures. There are few reports of good clinical outcomes using this technique.21,30 This option is more technically demanding than the above and results in few salvage options available if failure does occur. In addition, large volume implants have been shown to decrease the ability of angiogenesis and vascular ingrowth into the femoral head, thus increasing the rates of avascular necrosis (AVN).31 Furthermore, reports from the 1990s with high (20%) rates of nonunion and (15%) AVN raised concern for fracture site distraction using a blade in young patients with high-quality bone.32 As such, it is more commonly reserved for use in femoral neck nonunions, and there are insufficient data to support its use primarily in acute fracture fixation.
NOVEL TREATMENT ALTERNATIVES
Given the biomechanical advantages of fixed-angle constructs but high rates of shortening and functional impairment with uncontrolled sliding constructs, and catastrophic failures noted with length-stable locking plate fixation, alternative options for fixed-angle constructs have been developed. These “hybrid” fixation techniques combine the biomechanical benefits of fixed-angle constructs with the rotational stability of well-positioned MCSs. These options include the addition of a fibular strut graft to enhance support and fixation in the femoral neck, as well as novel dynamic compression laterally based locking plates.
MCSs/Pauwels Screw Augmented with Fibular Strut Allograft
Fibular strut grafts have been successfully used for nearly a decade to enhance fixation and improve both radiographic and clinical outcomes in proximal humerus fractures.33,34 More recently, this concept has been introduced to other fractures that are prone to subsidence, loss of reduction, or poor bony fixation including tibial plateau fractures35,36 and femoral neck fractures.37–39 One technique using a fibular strut allograft, described by Lorich et al40 in 2013, was developed with the benefits of fixed-angle length-stable fixation in mind. The technique also seeks to capitalize on the biomechanical benefits (increased stiffness and decreased fracture displacement) of widely spread inverted triangle cannulated screw constructs with a Pauwels screw placed perpendicular to the fracture.41 In this described approach, a Watson-Jones approach to the hip is performed to allow direct fracture reduction and implant insertion via the same incision.37 Alternatively, a Smith-Peterson approach for fracture visualization and reduction and separate laterally based incision can be used for implant insertion. Reduction is performed, followed by compression across the tension side of the fracture (inferior in valgus and superior in varus deformities) with a 7.3-mm partially threaded cannulated screw. A second partially threaded screw is inserted in a central position within the femoral head. Finally, a fresh frozen fibular allograft is inserted as an endosteal biologic dowel. First, the allograft is contoured on the back table to a core diameter of 10–11 mm. A guidewire is placed in the planned trajectory of the fibular graft, along the compression side of the fracture (superior for valgus and inferior for varus deformities), which allows control of the deformity in both coronal and sagittal planes. A 10- to 11-mm cannulated drill is used to create a channel for the graft and the allograft is inserted into the femoral neck and buried in the subchondral bone. The 2 partially threaded screws are exchanged for 7.3-mm fully threaded screws to provide a length-stable construct. Finally, a 3.5-mm cortical screw is inserted from the greater trochanter across the fracture to serve as a Pauwels screw. This Pauwels screw confers additional stability as demonstrated in biomechanical studies. By virtue of this screw also passing through the fibular graft into the femoral calcar, it also creates a fixed-angle construct between the native femoral neck and the allograft strut.37 Although no biomechanical studies have been performed comparing this construct with other fixed-angle constructs, the initial published clinical results of patients undergoing this technique have been promising.37 In this study, 27 physiologically young (18 less than 65 years) patients had an overall nonunion rate of 4% and 0% AVN rate at the mean of 19-month follow-up (range 12–30 months). Two (7.4%) catastrophic failures occurred in Garden IV fractures, one in a patient who was noncompliant with weight-bearing recommendations and the other in a patient who sustained a postoperative fall. The 1 nonunion and 2 catastrophic failures underwent conversion to total hip arthroplasty. All patients had a normal unassisted gait at final follow-up, and the cohort demonstrated excellent functional outcomes on the Harris Hip Score (average 91).37 A clinical example of this technique is shown in Fig. 1.
A 2015 randomized clinical trial performed in India compared 87 patients with displaced femoral neck fractures who received MCSs versus MCSs with a nonvascularized fibular autograft. Results of this study showed no benefit to fibular strut graft usage, with no difference in nonunion or AVN rates between groups (13% vs. 12% nonunion and 7% vs. 5% AVN for MCS and MCS with fibula, respectively).39 However, this study used only partially threaded screws and did not include a Pauwels screw through the fibula, thus it was neither a fixed-angle nor length-stable construct. The reported clinical success of the fibular strut allograft technique described above in comparison to the 2015 trial showing no difference is likely due to the increased stability of a fixed-angle and length-stable construct. Using this technique, the authors found no incidence of fibula or screw toggle or back-out after repeated construct cycling with hip motion. Postoperative MRIs performed at 3 months and 12 months revealed at least partial incorporation of the biologic strut into the host bone (57% at 3 months and 86% at 12 months).37 This spot welding phenomenon enhances the construct stability through the host bone–allograft interfaces over time, which is unique compared with conventional fixation methods that do not use biologic augments. The finding of osseointegration is unlikely to occur in constructs that do not allow stable, anatomic fixation at the fracture site, as well as Pauwels screw fixation through the graft.
To date, there are few alternative methods to enhance healing of intracapsular femoral neck fractures. Meyers introduced the concept of quadratus femoris muscle pedicle bone graft to enhance vascularity of the proximal fracture segment, and several authors have subsequently reported on results of femoral neck nonunions treated successfully with this adjuvant technique.42,43 A 2017 meta-analysis of 8 Chinese studies demonstrated improved healing rates and decreased rates of osteonecrosis with this technique, although each study demonstrated methodological flaws,44 and other researchers have been unable to replicate these favorable results in the acute fracture setting.45 In all, this approach has not gained widespread popularity over the past 3 decades, and positive results from further high-quality studies would be needed to support generalized use of this option.
Several studies attempting to revascularize the femoral head using a vascularized free-fibula autograft have been published.46 A recent randomized clinical trial of core decompression versus vascularized fibular grafting (27 patients with bilateral osteonecrosis, with 1 side serving as the internal control) revealed that the core decompression group had less favorable Harris Hip Scores at 18 months and more severe progression of osteonecrosis, staging at 36 months postoperatively.47,48 However, despite this early success, the long-term success of this technique is unknown. The application of free vascularized fibular autografting is impractical in the femoral neck fracture setting. The complexity and technical demands of free-fibula anastomosis and requirement of multiple surgical teams from different specialties to be present would lead to delayed surgery during which the vascular insult of fracture displacement and hematoma formation would remain unaddressed. Furthermore, this multidisciplinary approach overall remains impractical, if not impossible, for many hospitals in which these fracture patients are treated. Finally, the morbidity associated with fibular harvest and prolonged surgery to prevent a complication that occur in one-third or fewer of fractures is unjustified.
Although there is no widely accepted method of increasing fracture and femoral head vascularity and the optimal fixation construct has yet to be determined, the role of anatomic reduction and stable fixation in the intracapsular femoral neck fracture cannot be disputed. Despite increased soft-tissue dissection, there have been no reports of adverse effects of femoral head blood supply as a result of open reduction of a displaced femoral neck fracture. Therefore, surgeons treating this difficult condition must prioritize open reduction to achieve anatomic fracture reduction over percutaneous techniques that are more likely to yield unacceptable fracture alignment. Although there has been increasing interest in minimally invasive reduction and stabilization methods in other regions of the body as a method of preserving vascularity and cellular biology for healing at the fracture site, the authors believe that this approach should not be attempted for displaced femoral neck fractures because fracture reduction and compression in this biologically and biomechanically disadvantageous region will remain critical despite the choice of implants. Attempts to fix these fractures without appropriate appreciation for the importance of anatomic reduction will likely fail, leading to extensive revision procedures and/or arthroplasty. Future directions for investigation into the effect of bone morphogenic proteins in fractures that have been adequately reduced and stabilized may yield further insights into the healing potential of these fractures.
Displaced femoral neck fractures in young patients are fraught with complications including nonunion, AVN, and implant penetration through the femoral head. Implants used for osteosynthesis of these fractures must withstand physiologic hip forces without fatigue failure until fracture healing has occurred. MCS and sliding hip screw constructs are commonly used in the fixation of these fractures, however, each has biomechanical shortcomings. Two novel fixed-angle length-stable fixation options are presented that optimize biomechanical concepts including the use of MCS augmented with an endosteal fibular allograft and Pauwels screw, as well as fixed-angle dynamic compression locking plate, allowing controlled fracture compression. These promising options require further study before they can be recommended for widespread use.
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