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Biomechanics—Hot Topics Part I

Bottlang, Michael PhD*; Lorich, Dean G. MD; Dvorzhinskiy, Aleksey MD; Gausden, Elizabeth MD; Slobogean, Gerard P. MD, FRCS(C); Schemitsch, Geoffrey W. BKin§; Sanders, David W. MD, FRCS(C); Schemitsch, Emil H. MD, FRCS(C)§,‖

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Journal of Orthopaedic Trauma: March 2018 - Volume 32 - Issue - p S17-S20
doi: 10.1097/BOT.0000000000001123
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Proximal humerus fractures continue to pose a clinical challenge for orthopaedic surgeons. Recent studies comparing outcomes after proximal humerus fractures have put into question the benefit surgery provides over nonsurgical management, given the significant complication rates combined with mediocre clinical outcome scores after primary osteosynthesis.1–5 The 2 factors frequently implicated in the failure of osteosynthesis are the tenuous blood supply of the humeral head that can lead to avascular necrosis (AVN) and the profound osteopenia of the proximal humerus that results in an inability to obtain and maintain adequate fixation. The use of the anterolateral approach to the proximal humerus and incorporation of endosteal strut augmentation in combination with calcar support can address both of these issues and drastically improve results of osteosynthesis.

Although the ascending branch of the anterior circumflex humeral artery was historically considered the major perfusing vessel of the humeral head, more recent data have indicated that the medially based posterior humeral circumflex artery is equally if not more important to humeral head perfusion and that excessive medial dissection puts it at risk.6 Thus, protecting the posterior humeral circumflex artery and avoiding medial dissection is integral to the prevention of AVN. For this reason our group favors the anterolateral acromial approach over the deltopectoral approach for open reduction internal fixation of the proximal humerus. Furthermore, the anterolateral approach facilitates fracture reduction and application of a laterally based locking plate (LP).

Despite the importance of avoiding AVN after proximal humerus fractures, perhaps even more crucial to outcome is maintaining the reduction in osteoporotic bone. Although LP technology has led to improved results, loss of reduction and subsequent intraarticular penetration of the screws are still common complications.7,8 In a study of 35 proximal humerus fractures treated with LPs, both calcar reduction and calcar support were significantly associated with the maintenance of reduction and decreased screw penetration, whereas age and overall fracture pattern had no association with the outcome.9 The authors recommend placement of a superiorly directed oblique locked screw into the inferomedial portion of the proximal fracture because this is associated with the maintenance of reduction over time.

As the proximal humerus ages, there is an extension of the intramedullary canal region proximal to the physeal scar and increased vacuity within the humeral head, particularly in the greater tuberosity. The failed construct associated with proximal humerus osteosynthesis should be thought of as a failure of the bone rather than a failure of the implant. In cases of poor bone stock, we introduce afrozen allograft fibula that provides a structural, drillable, and biologic substrate. This creates a double-trestle construct that provides additional 2 cortices for screw purchase and decreases the working length of each screw that penetrates it. Use of the anterolateral approach with allograft fibula for endosteal support and calcar reduction produces outcomes superior to those reported in the comparison studies. Our series of 38 patients with complex proximal humerus fractures treated in this manner and followed for an average of 75 weeks found only 1 patient with partial necrosis of the humeral head and no radiographic evidence of screw penetration or cut out requiring removal and an average Constant score of 87 at last follow-up.10,11 In direct contrast to previous data, preoperative coronal deformity (varus/valgus) was found to have no effect on complication rates, radiographic, or clinical outcomes after surgery.12 A comparison of geriatric versus nongeriatric patients (mean age 74 vs. 53 years) found no difference in functional or radiographic outcomes at a follow-up of at least 12 months.13

Ultimately, successful outcomes for these fractures involve an appreciation for the anatomy and biomechanics of the proximal humerus. With these technical modifications, we are coming closer to achieving our goal of reestablishing preinjury levels of function for patients with proximal humerus fractures.


Femoral neck fracture (FNF) healing is fraught with complications because of the fixation challenges of these injuries. After internal fixation of a FNF, approximately 20% of patients experience a reoperation.14,15 These poor outcomes have fueled several treatment controversies, including the time to fixation, method of reduction, and the choice of fixation device.16–20

There are 2 main implant strategies used for FNF fixation: multiple smaller diameter cancellous screws or a fixed-angle device.16 Fixation with multiple cancellous screws has generally been the preferred method of fixation for most surgeons because of its rotational control of the femoral head and minimally invasive insertion. Although these benefits are achievable in many fractures, several surgeons have noted that a multiple screw construct typically fails through varus collapse, whereas a fixed-angle implant, such as a sliding hip screw (SHS) or cephalomedullary nail, can resist this failure mechanism.19

Patient bone quality, surgeon experience, and fracture characteristics should all be considered while selecting a FNF-fixation device. Among the injury characteristics to consider, the vertical nature of the fracture pattern, the degree of displacement, and the amount of fracture comminution are all associated with fixation outcomes.19,21,22 For undisplaced FNFs, nearly 80% of surgeons prefer to use multiple cancellous screws; however, when considering a patient with good bone quality and a displaced fracture, surgeons are equally divided on their use of multiple screws and fixed-angle implants.16

The biomechanical efficacy of different FNF implants has been compared in several experimental studies.23–26 The primary limitation of this existing literature is the variability in fracture patterns modeled, the implants compared, and the mechanical properties tested. Despite these inherent limitations, several consistent results seem to be observed: (1) none of the implants restore native mechanical properties24,25; (2) fixed-angle devices are stronger than multiple screws in load-to-failure testing23; and (3) fixation constructs can be created to resist the primary force(s) being tested.26 This latter observation is best demonstrated by a recent study reporting that augmenting multiple cancellous screws or a SHS with a 2.7-mm medial buttress plate increased failure loads by an average of 83%, with the multiple screws and buttress plate being stronger than the SHS and buttress plate.26

The “best” implant for FNF fixation remains controversial because of limited clinical data and limitations in biomechanical testing. Several small retrospective studies have suggested that decreased rates of fixation failure, nonunion, and femoral head osteonecrosis are achieved using fixed-angle implants.22,27 Although these reported benefits are encouraging, it is important to recognize that they contradict much older randomized control trials showing superiority of multiple screws over fixed-angle devices.28,29 Adding to the controversy, the recently completed fracture fixation in the operative management of hip fractures trial (n = 1108) demonstrated decreased composite reoperations with a SHS among displaced fracture patterns compared with using multiple smaller diameter screws; however, this difference was primarily driven by more implant removal procedures in the multiple screw group, whereas the SHS group experienced more femoral head osteonecrosis and conversion to joint arthroplasty.14 When one considers the entirety of available biomechanical and clinical data, it is clear that the optimal implant remains unknown; therefore, surgeons must apply a clear rationale to their implant selection based on their experience, desired fixation goals, and patient characteristics.


Modern surgical treatment options for periprosthetic distal femur fractures include the use of retrograde intramedullary nails (RIMNs), LPs, LPs with allograft strut (LP/allograft), and distal femoral replacement, although a consensus gold standard for surgical treatment has not yet been identified because of the lack of sufficient clinical and biomechanical evidence.30 Currently, surgical interventions using RIMNs and LPs represent the most common treatment methods for periprosthetic supracondylar femur fractures.

When compared with LPs, fixation using RIMNs is less invasive because the previous total knee arthroplasty incision can be reused. Therefore, no soft-tissue dissection is needed at the fracture site.31 Patients can also begin range of motion and weight-bearing exercises immediately, which lowers the risk of arthrofibrosis development. In comparison, patients treated with LP fixation often delay starting weight-bearing for 6–12 weeks postsurgery.32 Biomechanically, the use of RIMN has been associated with significantly lower rotational stability under torsional loading when compared with LP. Conversely, RIMN fixation has shown significantly higher load to failure under axial stress.33 A study conducted by Chen et al34 found that RIMN and LP fixations were comparable in terms of fracture stabilization and construct stiffness; however, both constructs were significantly inferior to the use of LP/allograft fixation. Although LP/allograft fixation constructs have shown biomechanical advantages, their use is accompanied by increased cost, surgical complexity, potential for disease transmission, and limited availability.

There are technical factors which may make RIMNs less desirable. Some issues with RIMNs include the need for adequate distal bone stock for locking screws, thus eliminating the use of RIMNs in extreme distal cases. RIMNs should not be used in the presence of ipsilateral total hip arthroplasty to avoid fracture between the 2 implants due to the creation of a stress riser.31 In addition, RIMN fixation may be unsuitable in highly comminuted or length-unstable distal fracture patterns.32 Finally, in some instances, knee arthroplasty femoral components may have a closed box, which eliminates the potential for insertion of the RIMN.

Modern LP procedures have become less invasive with minimized periosteal stripping and soft-tissue dissection when compared with previous procedures; however, a separate lateral incision is still required, which may increase patient morbidity.35 LPs are advantageous in the presence of preexisting proximal femur hardware and in highly comminuted and length-unstable distal fracture patterns.32 LPs allow for improved fixation with small distal fragments because of the presence of multiple potential distal fixation points.

Generally, available clinical evidence is limited, but it suggests that fixation using LPs and RIMNs is comparable in terms of postoperative outcomes. A systematic review of 415 fracture cases by Herrera et al36 showed that the use of RIMNs was associated with lower but nonsignificant rates of nonunion, secondary infection, and hardware failure when compared with LPs. A similar systematic review by Ristevski et al (2015) which examined 719 eligible fracture cases found that the use of LPs yielded higher but nonsignificant rates of nonunion, with rates of 3.6% and 8.8% reported for RIMNs and LPs, respectively. However, the use of RIMNs was associated with a malunion rate of 16.7%, which was significantly higher than the rate of 7.6% for LPs.37 Finally, a review performed by Ebraheim et al (2015) examined differences between LP and RIMN fixations in the healing rate and complication rate of 488 and 448 fractures, respectively. In terms of healing rate, LP and RIMN constructs were comparable with healing rates of 87% and 84%, respectively. Differences were found in the complication rate because LP constructs exhibited a lower complication rate of 35% when compared with 53% for fixation using RIMN devices.38

In conclusion, high-grade evidence is lacking when comparing the use of LPs and RIMNs for fixation of these fractures. Thus, a prospective randomized control trial is warranted and would provide much needed evidence to help identify a gold standard for treating these patients.


Designing the ideal syndesmosis implant should provide the correct degree of both stability and flexibility. Stability should be sufficient to withstand active motion and potentially axial loading, with no decay in strength while healing occurs. Similarly, flexibility should be sufficient to allow full ankle motion. Fortunately, a substantial amount of work by many researchers has provided insights into issues of both stability and flexibility.

To determine the ideal stability, Stauffer et al39 performed force and motion analysis of ankle joints and noted that the degree of axial compression during gait is approximately 5 times the normal body weight. Assuming that the fibula carries approximately 10% of the load, and perhaps slightly more in dorsiflexion, leads to an estimate of approximately 500 N loads during gait. Burdett et al40 in a later analysis using force platform analysis of cadaveric ankles predicted much higher loads during running—a multiple of 2.5 compared with walking or approximately 1250 N. Torque resistance is somewhat more difficult to determine compared with the axial load. Previous patient-based studies suggest that exceeding 5–10 Nm of torque causes significant discomfort at the syndesmosis, and therefore, 7.5 Nm has been suggested as a reasonable degree of torque resistance.41

With respect to flexibility, the normal tibiofibular joint exhibits a small but important degree of motion during tibiotalar flexion and extension. As the ankle dorsiflexes, the mortise widens 1–2 mm, and the fibula externally rotates 3–5 degrees. Although we think of the fibula moving proximally in dorsiflexion to accommodate the widened anterior talus, it is actually being pulled inferiorly approximately 2.4 mm during weight-bearing by the foot flexors.42 This “functional lengthening” of the fibula deepens the mortise and tightens the interosseous membrane, resulting in more lateral support during stance and push off. Rigidly fixing the fibula at neutral length may therefore paradoxically decrease lateral stability.

Current implants include screw fixation and the flexible fixation techniques such as the TightRope device (Arthrex, Naples, and FL). Most biomechanical testing applies an axial load of approximately 750 N and an external rotation torque of 7.5 Nm. Researchers including Soin et al detected higher failure torque with screw fixation compared with the TightRope, but the failure torque exceeded normal forces (26 vs. 23 Nm).43 Other authors noted equivalent pullout strength44 and slightly more diastasis with the suture-based implant,45 with failure typically noted at loads well in excess of 500 N. In summary, mechanical testing has found that both fixation strategies provide stability, which substantially exceeds the typical forces generated with active motion. Much less data are available to guide implant selection based on flexibility; however Soin et al assessed motion comparing the TightRope and a 3.5-mm quadracortical screw and found that neither of the constructs reproduce normal ankle motion.43 Clinical studies may favor the use of the TightRope for restoration of ankle motion in the short term but again data are limited.

Clinical studies demonstrate that both implant strategies are likely successful in restoring stability while permitting adequate flexibility of the tibiofibular syndesmosis. Cottom et al46 found no difference between the fixation strategies at 6 months. Laflamme et al performed a comparative randomized controlled trial between percutaneous screw and TightRope fixation and noted that clinical outcomes were similar in both groups comparing the American Orthopaedic Foot and Ankle Scores. By contrast, Olerud–Molander ankle score was slightly improved in the TightRope group at 3, 6, and 12 months, but the difference averaged only 6 points on a 100-point scale. Final outcomes were excellent in both groups.47 Ongoing studies, including a randomized controlled trial funded through the Orthopaedic Trauma Association, should provide further information to add to the discussion.

The ideal fixation technique for the tibiofibular syndesmosis should provide ample stability while permitting full flexibility of the ankle joint. The likelihood is that obtaining and maintaining an accurate reduction is the most critical factor in syndesmosis repair; the fixation strategy is probably secondary because both implants seem acceptable.


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Biomechanics; fracture fixation; humerus; femoral neck; distal femur; syndesmosis

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