An estimated 34,000 hip fractures occurred in patients in the United States in 1996,1 and approximately 1.7 million hip fractures worldwide occurred that year. Between 1986 and 2005, the annual mean number of hip fractures was approximately 950 per 100,000 for women and 400 per 100,000 for men.1 As life expectancy increases, the number of patients over the age of 65 will continue to grow, and thus, it is expected that the burden of disease for geriatric hip fractures will substantially increase over time.2 Most intertrochanteric hip fractures are treated with reduction and surgical fixation. In general, the types of fixation can be broken down into intramedullary (IM) and extramedullary,2,32,3 with extramedullary fixation consisting largely of plate and screw constructs for the proximal femur. This article will review varying types of IM and extramedullary fixation, evaluate existing biomechanical models, and discuss the application of fracture models to clinical practice.
IM NAILING OF PROXIMAL FEMUR FRACTURES
Proximal femur fractures have traditionally been treated with the use of either an IM nail or an extramedullary sliding hip screw (SHS). Because of the devastating nature of proximal femur fractures, the efficacy of the differing treatment options must be assessed, especially when treating unstable fractures or when signs of comminution, osteoporotic bone, or pathologic bone are present. Treatment has trended toward the use of IM nails versus plating and SHSs. A study by Anglen and Weinstein4 examined the relative use of plates or nails during fixation of intertrochanteric femur fractures from 1999 to 2006. The results indicated an increase in the use of nails during fixation from 3% to 67% and a subsequent decline in the use of plates from 97% to 33%.4
This rise in nailing can be attributed to a number of factors, including certain biomechanical advantages when compared with SHSs, especially during fixation of unstable fractures like transtrochanteric, reverse oblique, or subtrochanteric extension fractures. These biomechanical advantages may translate into patient-important outcomes, such as reduced reoperation rates. For example, a study by Matre et al5 examining 2716 intertrochanteric fractures found that patients who were treated with SHSs had a 43% higher risk of reoperation. Furthermore, IM nailing was favored when patient pain, satisfaction, quality of life, and mobility were examined. A study by Sadowski et al6 examined the treatment of 39 unstable proximal femur fractures with either an IM nail or a 95 degree screw-plate in elderly patients. Implant failure was observed in 7 of the 19 patients who were treated with the screw-plates compared with 1 of 20 in the group treated with IM nails. Shorter hospital stays were also observed in a study by Bohl et al,7 which examined the use of either IM nailing or an extramedullary implant on 4432 patients with intertrochanteric femur fractures. This decreased hospital stay may offset the higher cost of an IM nail. Swart et al8 furthermore elucidated the cost-effectiveness of IM nailing in unstable proximal fractures when accounting for the hardware failure rate.
Modern nails are advantageous in that they are smaller, have high cutout resistance, excellent lag screw stability, and dynamization capability. The following are recommendations to maximize the effectiveness of IM nailing in proximal femur fixation. First, the correct entry point must be obtained and proper reduction achieved before reaming. The implant construct should be optimized. With simple, well-reduced fractures, the choice of implant is not critical. With unstable fractures, the implant that allows minimal fracture site motion should be used.9The lag screw should be placed in a center/center position or center/inferior position in the head with static locking,10,1110,11 and the length of the nail does not seem to make a difference.12
The initial forms of internal fixation for geriatric hip fractures consisted of fixation constructs using plates with fixed angle nails or blades in the proximal femur.13 These techniques gradually transitioned to the use of an SHS. In 1964, Clawson13 described the treatment of intertrochanteric hip fractures with an SHS fixation method. In this work, he reported that “stable” fracture patterns did well with fixed angle nail-plate devices, but that unstable fractures continued to have problems. An improved fixation has been demonstrated with sliding hip devices when compared with traditional fixed angle implants.13,1413,14 Valgus alignment of the initial reduction of intertrochanteric hip fractures has been shown to be beneficial, as intertrochanteric fractures often tend to collapse into a stable position with healing, and the SHS therefore allows for a controlled collapse of the fracture, maintaining length and a functional abductor mechanism after healing.3,133,13
Baumgaertner et al14 described the importance of the compression screw position to the overall stability of intertrochanteric hip fractures, indicating that the screw needs to be placed in a center–center position on both the AP and lateral views and quite deep in the femoral head. Haidukewych et al15 reported that reverse obliquity intertrochanteric fractures have a much more successful rate of union and lower loss of reduction with IM fixation as compared with plate fixation. More recently, Palm et al16 identified that the integrity of the lateral wall of the proximal femur is important in identifying potential failure with an SHS. Specifically, lack of continuity of the proximal lateral wall of the femoral shaft was found to allow the proximal fragment to collapse excessively laterally, leading to substantial shortening of the abductor mechanism. The lateral wall effect can be counteracted with a trochanteric buttress plate or stabilization plate to prevent excess collapse.15,1715,17 However, this technique has been found to be, at best, equivalent to or slightly less effective than IM fixation.
The primary factors under the control of the surgeon treating patients with intertrochanteric hip fractures are the quality of the reduction obtained during surgery and the placement of the implants. Often, closed reduction techniques are used to reposition the proximal femoral fragment relative to the shaft. However, there are times when, because of comminution or significant soft tissue tensions, the alignment of the fragments may not be ideal to allow for stable compression leading to well-positioned fracture healing. In these cases there may be need for adjunctive open reduction at the time of internal fixation. Several authors have described the benefit of performing a manual assessment of the fracture alignment of the anterior cortex of the femoral shaft with the anterior portion of the intertrochanteric fragment ensuring that there is bony contact and stability at the time of fixation.18–2018–2018–20 Furthermore, care must be taken at the time of implanting the fixation device to not alter the alignment of the fracture fragments and to prevent rotational or angular displacement. Sommers et al used a laboratory model to simulate the clinically-seen failure mechanism of screw cutout from the proximal head and neck portion of the intertrochanteric fracture fragment.21 In this work, the authors observed that rotational displacement of the proximal fragment, specifically into varus and extension as observed clinically, was responsible for the cutout failures seen in biomechanical tests. Because this observation, the importance of rotational stability of the fracture fragment has been emphasized. Varying derotational devices consisting of screws and side plates have therefore been introduced to provide a more stable rotational environment for the intertrochanteric fragment.3,22,233,22,233,22,23
The results of several prospective, randomized trials that compare plate fixation to nailing are shown in Supplemental Digital Content 1 (see Table, http://links.lww.com/BOT/A554).22–2622–2622–2622–2622–26 Generally, these data demonstrate little to no difference in terms of overall complications and the medical aspects of care. Overall, healing rates are similar between IM and plate-based constructs in studies that consider 31-A1 and 31-A2 fractures.18 An improvement was observed in healing with maintenance of the reduced position for IM fixation, particularly for the more unstable fracture patterns.
The results of studies that assess different types of plate fixation for geriatric intertrochanteric fractures have been compiled in Supplemental Digital Content 1 (see Table, http://links.lww.com/BOT/A555).19,20,2719,20,2719,20,27 The percutaneous compression plate (PCCP) and the compression hip screw have been shown to have similar rates of overall complications, but the PCCP exhibited evidence for an improved maintenance of the reduction position.
A recent Cochrane review compared different plate fixation methods for intertrochanteric femur fractures. Fixed angle plates, including the Jewett fixed nail type and the Resistance Augmented Bateaux plate, were compared with SHSs, PCCPs, and other devices.3 The authors concluded that fixed nail plates had a greater fixation failure rate when compared with SHSs, and thus, SHSs continue to be the overall preference for intertrochanteric fracture fixation.
Plate fixation remains an important type of implant for stable intertrochanteric hip fractures. Importantly, the cost of an SHS is substantially less than the cost of an IM device for fixation of the same fracture.28 Indeed, studies with devices such as the PCCP and other rotationally stable plating systems may show an improved overall final position at the time of fracture healing. However, data demonstrating that these rotationally stable implants also improve quality of life and reduce revision rates is lacking, and thus, they may not warrant the added costs associated with their use. Proactively managing costs and quality of care is of increasing importance,2 as surgeons must consider both the quality and value of the care they deliver. Better data are therefore needed to inform orthopaedic surgeons as to what treatment options result in both improved quality and increased overall value for patients.
SELECTING THE BEST MODEL: COMPARING EXISTING DEVICES TO NEW CONSTRUCTS
Two fragment OTA 31-A1, 2, and 3 type fractures are inherently stable once reduced and fixed. OTA 31-A2.1, 2, and 3 fractures, all having more than 2 fragments, are difficult to reduce, often have a compromised medial buttress, and are inherently unstable. Most fracture models aim to address A2 type fractures, because they result in the most biomechanical failures clinically. The most common mechanical failure modes for fixed nail-plate devices include head cutout, varus angulation, screw loosening or pull out, and nail-plate junction failure. Failure modes for sliding nail-plate devices include excessive collapse, varus angulation, screw loosening, and less often head cutout if the sliding mechanism jams. IM devices also can have head cutout if the sliding mechanism of the head and neck screws jam in the nail. Excessive collapse, varus angulation, and fracture of the femoral shaft in the subtrochanteric region can occur at the distal cross screw.
These failure modes can be duplicated with a single compression load on the proximal femur in cadaver bones with 3 or 4 part OTA 31-A2.1-3 type fracture patterns. Fresh frozen human femurs make the best model, and formalin fixed femurs have been used successfully. Some synthetic bone models may work well but must be validated.
Most models try to simulate the compression load at heel strike, which is typically the peak load seen during activities of daily living. With most fracture models, the expected clinical failure modes can be duplicated with this simple loading. For other loading conditions one can visit Orthoload.com to see the magnitude and direction of loads on the hip for a wide variety of activities. Regardless of the magnitude and alignment of the chosen load, the end conditions for the bone-fracture-fixation construct are critical to properly simulate the behavior seen clinically. The femoral head must be able to rotate in any direction as it would in the acetabulum. Varus rotation is the most common rotation seen clinically. Most testing machines can apply an axial compression load in the vertical direction, so the distal femur must be aligned to vertical at the angle of the desired applied load. The proximal or distal boundary condition must either allow both fragments to rotate as load is applied (Fig. 1A), or if one is held in a fixed angle to vertical, it must be allowed to translate (Fig. 1B).29,3029,30 Both of these models work well, as they allow the fractured fragments to rotate relative to each other as would occur in a live subject.
If the objective is to simulate a sudden type of mechanical failure, a single load can be applied to the model until failure is observed. If loosening or fatigue is the desired failure mode, cyclic loading can be applied. A single direction of load at a single point aligned to the femoral head is often quite effective. Adding an abductor load creates a more realistic moment at the hip joint but is often impractical with intertrochanteric fractures because the abductor load must be applied to the greater trochanter, which can be very weak and may not tolerate the load. With robotic load application one can achieve very complex loading cycles to simulate a variety of activities. For instance, in rare cases the sliding screw has pulled out of the barrel of the nail plate. This type of failure cannot occur with the simple loading described above, but certain activities, such as getting in or out of bed, have shown a reversing component of the load on the femoral head that might tend to pull the screw from the barrel (see Orthoload.com).
The other issue important to fracture fixation is the quality of bone. Mechanical failures are much more prevalent in osteoporotic bone than in normal bone in young, healthy subjects. Unfortunately, most of these very unstable fractures occur in the more elderly population, in which osteoporosis is more prevalent. Most of these unstable fracture models have shown that if the medial cortex can be aligned at the time of fixation, the fracture will behave in a stable manner.31,3231,32 However, the fixation must be sound enough to keep the bone fragments reduced. Cancellous bone in the femoral head is strong enough to accomplish this in normal bone, but in an osteoporotic femoral head and neck it is much sparser, unevenly distributed, and usually absent in the very location where most screws tend to end up after surgical fixation. It is therefore very difficult to prevent slip of the medial cortex of the proximal fragment off the medial cortex of the distal fragment, which results in the proximal fragment rotating into varus. This common occurrence can be modeled well with osteoporotic human femurs but not with current synthetic femur models.
The development of an appropriate biomechanical model begins with careful definition of the clinical problem to be addressed, followed by appropriate choice of materials, boundary conditions, and loading conditions to mimic the clinical situation. One must then validate the model before planning the definitive study sample size and outcome measures.
TRANSLATING BIOMECHANICAL STUDIES INTO CLINICAL PRACTICE
Bones have the unique ability to repair structural failure with the resorption and creation of new bone into a structurally equivalent functional construct. Conversely, human biomaterials do not yet possess self-regeneration abilities, and thus, bone regeneration in the fracture zone is the only hope for survival of the implant. Anything man-made will fail in an unstable fracture or bone defect environment; it is only an issue of how, when, and why. Biomechanical studies to determine the modes and time to failure of implants have been developed in an attempt to forecast survival rates in hip implants. The Smith-Petersen Nail designed in 1925 and combined with an open anatomic reduction was the first globally successful device for hip fracture fixation. With widespread use of this technique and device, new failure modes were discovered, including electrogalvanic corrosion.33,3433,34 Smith35 in 1953 first reported mechanical testing involving the creation of femoral neck fractures and the loads to create fractures. Building on this information, Martz and Foster pioneered the load to failure model for hip fracture testing, estimating the loads required for a hip fracture implant to fail.36,3736,37 Unfortunately, this focus on implant survivability diverted attention away from surgical reduction and the functional recovery of the patient.
In 1963, the theoretical modeling of fracture implant stability took a wrong turn when Holt suggested that rotational forces were unimportant in pertrochanteric fractures, and this single article resulted in 40 years of stasis in hip fracture biomechanical modeling.38 In 2004, Sommers et al developed a new biomechanical model for hip fracture stability, demonstrating the importance of rotation in hip screw cutout, the most important mode of failure for hip fracture fixation.21
Ehmke et al39 renewed the appreciation of multiplanar loading vectors on hip fracture fixation and implant migration, as proposed by Smith-Petersen 80 years earlier. This article helped stimulate investigation of another form of implant failure affecting patient functional outcome: that of fracture collapse and shortening of the hip. This complication was largely ignored as a sacrifice to prevent implant failure. In 2008, Zlowodski et al40 demonstrated in a multicenter study the negative effect of excessive shortening of the hip.
Another aspect of hip fracture modeling is the quality of the patient's bone. Insufficient bone quality is one aspect of fracture repair that often justified arthroplasty replacement surgery. New calcium phosphate cements with intrusion properties combined with augmented implant designs present the potential to modify the material properties of the next generation of hip implants.41
New biomechanical models will continue to be required to reflect clinically relevant issues and to optimize the current generation of hip fixation systems. The ultimate goal should be the ability for a hip fracture patient to expect the same functional recovery as elective total hip arthroplasty patients.
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