Biomechanical Concepts for Fracture Fixation

Abstract

Application of the correct fixation construct is critical for fracture healing and long-term stability; however, it is a complex issue with numerous significant factors. This review describes a number of common fracture types, and evaluates their currently available fracture fixation constructs. In the setting of complex elbow instability, stable fixation or radial head replacement with an appropriately sized implant in conjunction with ligamentous repair is required to restore stability. For unstable sacral fractures, “standard” iliosacral screw fixation is not sufficient for fractures with vertical or multiplanar instabilities. Periprosthetic femur fractures, in particular Vancouver B1 fractures, have increased stability when using 90/90 fixation versus a single locking plate. Far Cortical Locking combines the concept of dynamization with locked plating in order to achieve superior healing of a distal femur fracture. Finally, there is no ideal construct for syndesmotic fracture stabilization; however, these fractures should be fixed using a device that allows for sufficient motion in the syndesmosis. In general, orthopaedic surgeons should select a fracture fixation construct that restores stability and promotes healing at the fracture site, while reducing the potential for fixation failure.

Introduction

Selection of the appropriate fracture fixation method is a multifaceted issue that depends on the location and type of fracture, the inherent stability at the fracture site, and the desired degree of flexibility and motion required for healing. This review assesses current fixation devices used in radial head and coronoid fractures, sacral fractures, periprosthetic and distal femur fractures, and syndesmosis injuries, and discusses the biomechanics of each.

Radial Head and Coronoid Fractures: Biomechanical Evidence for Modern Approaches

Several biomechanical studies have provided important evidence that can help to guide the management of radial head and coronoid fractures of the elbow. The majority of these have focused on the importance of appropriate management of these fractures when combined with ligamentous injury (i.e. complex elbow instability/fracture-dislocations of the elbow). From these studies, several noteworthy concepts can be obtained.

First, stable fixation or replacement of the radial head in the setting of traumatic elbow instability is critical. Multiple biomechanical investigations have demonstrated that the radial head is critically important to elbow stability when the ligaments of the elbow have been disrupted.^1-3 These studies have further shown that elbow stability is best restored by ligamentous repair combined with stable radial head fixation or replacement. This biomechanical literature, combined with clinical evidence which has shown that significantly displaced radial head fractures are an important predictor of elbow dislocation,⁴ suggests that these fractures are best treated with stable fixation or replacement in combination with ligamentous repair. Practically speaking, this means that radial head excision is contraindicated in the setting of complex elbow instability, and that if stable fixation cannot be achieved, radial head replacement should be performed. This, combined with the clinical evidence of poor outcomes in comminuted, displaced radial head fractures treated with open reduction and internal fixation (ORIF),⁵ has led to a distinct shift by orthopaedic trauma surgeons toward radial head replacement in the setting of complex elbow instability, even in young patients.

Second, correct sizing of a radial head replacement is critical to restoring joint kinematics and contact pressures, and yet improper sizing is one of the most commonly encountered surgical errors. This is typically due to overstuffing of the joint in an effort to compensate for instability of the elbow. Biomechanical studies have demonstrated that improper sizing of the radial head replacement results in increased joint contact pressures, elbow instability, and altered biomechanics in the forearm.^{6, 7} These studies have shown that the best restoration of elbow stability and mechanics occurs with accurate radial head sizing and ligamentous repair. Several anatomic and radiographic landmarks for accurate radial head sizing have been described in the literature and should be used accordingly.⁸

Third, stable fixation of a complete radial head fracture is best achieved with crossed screws or a fixed angle device, particularly in the setting of incomplete cortical contact between the head and neck. Two separate biomechanical studies have shown the importance of using such constructs to obtain stable radial head fixation in the setting of a complete articular fracture of the radial head.^{9, 10}

Finally, when considering the need for fixation of the coronoid in the setting of complex elbow instability, both the size and location of the coronoid fragment is important. Several biomechanical studies evaluating the influence of coronoid fragment size and fixation in the setting of terrible triad injuries have shown that it is not necessary to perform fixation of coronoid fragments involving less than 40-50% of the coronoid, provided that radial head stability is restored and ligament repair is performed.^11-13 This literature suggests that previous thinking, which recommended that coronoid fragments of any size be fixed in the setting of terrible triad injuries, was incorrect. Subsequent clinical literature has provided further support for this notion.¹⁴ Biomechanical research has shown that the location of the coronoid fracture is also important in elbow stability, as even small anteromedial facet fractures (>5mm) can be significantly unstable and likely warrant fixation.¹⁵

Unstable Sacral Fractures: Is Standard Iliosacral Screw Fixation Adequate?

Unstable pelvic ring injuries usually result from high-energy traumatic events such as automobile accidents, and in osteopenic patients due to less violent incidents such as falls. Sacral fractures are common in pelvic ring injuries, and their instability depends on numerous factors, foremost among them the magnitude of the applied traumatic load.

Iliosacral screw insertion became a popular fixation method for unstable posterior pelvic ring injuries as they could be safely inserted via small surgical incisions, which lowered wound complication rates, operative blood losses, and operative times significantly, while avoiding deep pelvic hematoma.¹⁶ Furthermore, high quality intraoperative fluoroscopic imaging was available to guide and assess closed posterior pelvic reduction techniques and iliosacral screw insertions. “Standard” iliosacral screw fixation was commonly defined as a single cannulated 7mm screw inserted from the lateral iliac cortical surface, across the sacroiliac (SI) joint, through the sacral ala below the alar cortical surface and above the nerve root tunnel, and ending in the upper sacral vertebral body or contralateral ala (Supplemental Digital Content Figure 1A). The screw starting point, directional aim, and length were determined by the particular injury pattern and osteology. The development of longer, 7mm diameter cannulated cancellous screws allowed for transiliactranssacral (TITS) screw orientations to be used (Supplemental Digital Content Figure 1B). One clinical study showed that fixation failure rates were significantly lowered in patients with unstable posterior pelvic injuries when TITS screws were used compared to shorter length screws.¹⁷ Patient compliance must also be considered when planning the sacral fixation construct, as early unprotected weight-bearing after surgery can lead to fixation failure. “Standard” iliosacral screw fixation alone is therefore not advocated for potentially noncompliant patients.

In several clinical series, a “standard” iliosacral screw in the upper sacral segment fixation construct failed to maintain the reduction, especially with certain injury patterns.^{16, 18} The most common factor associated with “standard” fixation failure was a highly displaced sacral fracture that had been poorly reduced, either after closed manipulation or open reduction. Poor reduction of a sacral fracture was also noted to diminish the safe region available for iliosacral screw insertion.¹⁹ Unsurprisingly, the more unstable fracture patterns also correlated with higher failure rates.¹⁸ Another factor noted to increase failure in clinical practice was insufficient stabilization of the other pelvic ring injury sites. This correlates with biomechanical studies, which found that reduction and fixation of each injury site improved the overall fixation construct strength.²⁰ Transverse sacral fracture patterns require additional consideration with regard to iliosacral screw fixation. U-shaped sacral fractures cause instability of the entire spine, along with the upper sacral segment and the rest of the pelvis. U-shaped fractures exclude overall ring involvement, but fixation failures have occurred in association with standard iliosacral screw fixation.²¹ Thus for U-shaped sacral fractures, TITS screws located in the safe sacral osseus fixation pathway, cranial to the transverse fracture, provide stable and durable fixation. Y- and H-shaped sacral fracture patterns are U-shaped fractures that are further complicated by associated pelvic ring injuries. For these injuries and U-shaped fractures that require spinal decompression due to cauda equina syndrome, supplementary spinopelvic fixation should be considered.

Based on clinical and biomechanical studies in addition to clinical experience, “standard” iliosacral screw fixation (one upper sacral segment screw) is more likely to be a successful treatment for patients with rotationally unstable injuries, especially when the anterior pelvic injury and the sacral fracture are accurately reduced and stabilized. For more unstable sacral fracture patterns with vertical or multiplanar instability, “standard” iliosacral screw fixation will not provide adequate stability. Following accurate sacral fracture reduction, several iliosacral screws of appropriate lengths, located at multiple sacral levels are optimal. Other important factors that contribute to overall pelvic ring stability and the durability of the fixation construct include anterior pelvic injury reduction quality and the choice of implant. Poor anterior pelvic reductions and less stable anterior pelvic fixation devices, such as external fixation, place additional stress on the sacral fracture fixation construct. Lumbopelvic fixation can also be used to supplement iliosacral screw fixation.

Periprosthetic Femur Fractures: 90/90 Fixation Versus a Single Locking Plate?

Periprosthetic femur fractures are a complication of total hip arthroplasty, and although uncommon, they are increasing in frequency with the aging population.^{22, 23} Classification of periprosthetic femur fractures is based on the Vancouver System.²⁴ The focus of this review is Vancouver B1 fractures, which account for a majority of periprosthetic femur fractures.^{22, 25} Vancouver B1 fractures occur around or just distal to a stable prosthesis. Correctly identifying the fracture type is imperative to providing the best chance for a successful outcome.^{24, 26} Vancouver B1 fractures are particularly difficult to treat as they have a high complication rate and the proper treatment method is still under debate.^{23, 27}

A 90/90 fixation construct for a periprosthetic femur fracture involves a plate placed laterally and a cortical allograft strut placed anteriorly. Strut allografts allow greater mechanical stability and increase bone stock, leading to improved fracture healing.^{28, 29} Benefits of using a locking plate alone include a minimally invasive technique of insertion, increased angular stability, and a decreased need for plate contouring.^{23, 28}

Biomechanical studies have indicated that a lateral cable plate with an allograft strut placed anteriorly may provide superior fixation for periprosthetic femoral fractures. A study by Zdero et al. showed that 90/90 fixation achieved equal or superior results in axial stiffness, lateral bending stiffness, and torsional stiffness tests compared to locking plates alone.²⁷ Moreover, the results seen with the single locking plate constructs were similar to non-locked cable plates. These results were maintained with cyclic loading. A locked plate alone was less stiff in bending and had a lower load to failure than 90/90 allograft strut-plate constructs with or without locking screws.³⁰

A systematic review conducted by Dehghan et al. compared different operative treatments for Vancouver B1 periprosthetic femur fractures.²⁶ Two of the treatments that were compared included ORIF with cable plates and cortical strut allografts and ORIF with locking plates. This systematic review indicated that locking plates had a higher rate of nonunion and hardware failure compared to a cable plate with a cortical strut allograft (90/90 fixation construct).

Both the biomechanical evidence and systematic review suggest that 90/90 fixation yields superior results compared to a single locking plate and should be used in the treatment of patients with a Vancouver B1 periprosthetic femur fracture when maximum rigidity is required. In particular, a locking plate should be avoided if there is no medial cortical contact at the fracture site and with transverse fractures.

Distal Femur Fractures: Far Cortical Versus Conventional Locking Screws – Is there a New Gold Standard?

Far Cortical Locking (FCL) is a strategy to dynamize a locked plating construct in order to promote biological fracture healing by callus formation.³¹ Applying a locking plate with FCL screws instead of conventional locking screws reduces the construct stiffness and enables controlled axial motion that leads to faster and stronger fracture healing.^{32, 33}

The original gold standard for flexible fracture stabilization promoted callus formation using elastic fixation constructs, such as intramedullary nails, external fixators, and functional bracing.^{34, 35} This flexible fixation strategy is supported by over 50 years of research demonstrating that controlled dynamization of a fracture promotes callus formation and improves the speed and strength of fracture healing.^36-40 For example, Goodship and Kenwright demonstrated that 1 mm axial dynamization delivered over three times stronger and two times faster healing compared to rigid fixation.³⁷ Conversely, deficient fracture motion caused by overly stiff fixation constructs can suppress secondary fracture healing, contributing to delayed union, nonunion, osteolysis, and fixation failure.^{41, 42}

Locking plates can provide stronger and more durable fixation than non-locked plates. However, locked plating constructs are also inherently stiff and can suppress motion at the fracture site to levels insufficient for stimulation of callus formation.^{41, 43} Consequently, locked plating of distal femur fractures causes deficient and asymmetric callus formation, with the least amount of callus being deposited at the near cortex.^{41, 42} This concern is corroborated in recent studies on locked plating of distal femur fractures, documenting nonunion rates of 10-23%,^44-47 and a 31% re-operation rate for open fractures.⁴⁸

Yet with the advent of locked plating came novel strategies for dynamization,^{31, 38, 49} since locking plates derive stability from fixed-angle locking screws and thus no longer require plate compression onto the bone surface. The strategy of FCL enables controlled and symmetric interfragmentary motion through elastic flexion of screw shafts within a motion envelope at the near cortex.³¹ A biomechanical study demonstrated that FCL screws enable axial dynamization without sacrificing construct stability.³² In an ovine fracture healing study, FCL constructs delivered consistent and circumferential callus bridging and yielded 157% stronger healing compared to standard locked plating.³³ Clinically, a prospective study of 31 consecutive distal femur fractures stabilized with FCL constructs reported no implant or fixation failure, an average time to union of 16 weeks, and a nonunion rate of 3%.⁵⁰ The FCL strategy has been implemented in commercial implants (MotionLoc FCL screws, Zimmer; Dynamic Locking Screws, Synthes), and has been clinically employed for fixation of distal femur fractures,^50-52 tibial fractures,⁵³ and humeral fractures.⁵⁴ FCL fixation has also been simulated using standard locking screws by means of overdrilling⁵⁵ or slotting⁵⁶ of the near cortex. Moreover, “active” locking plates have been developed that provide controlled axial dynamization through elastic suspension of the locking holes within the plate, while using standard locking screws.⁴⁹ However, to date, only FCL screws have been fully evaluated by bench-top and cadaveric testing,^{32, 57} a prospective randomized animal study,³³ and clinical trials.⁵⁰ Alternative strategies for dynamization should therefore be viewed with caution until they have been fully evaluated.

In conclusion, FCL combines the superior fixation strength of fixed-angle locking screws with controlled dynamization to promote biological healing. Given the established benefits of dynamization and the evidence that stiff locking constructs can suppress healing, a new standard for locked plating of distal femur fractures should account for dynamization to promote healing.

Syndesmotic Injuries: What is the Ideal Fixation Construct?

Physiological motion at the syndesmosis is complex. Normal motion at the ankle necessitates translational, rotational, and migrational movements of the fibula at the syndesmosis.⁵⁸ During plantarflexion, the fibula migrates distally, translates anteromedially, and internally rotates. Dorsiflexion results in migration of the fibula proximally, posterolateral translation, and external rotation. External rotation of the foot causes a medial translation, posterior displacement, and external rotation of the fibula through the syndesmosis.⁵⁹ Screw fixation alters fibular translation and rotation.^60-62 As a result, syndesmotic screw fixation has been shown to restrain the mortise-width-variations during foot dorsi- and plantarflexion with reduced range of motion for horizontal translation.⁶³

Likewise, syndesmotic screws cannot prevent syndesmotic widening when subjected to weight bearing.⁶⁴ Rigid fixation of the syndesmosis restricts normal physiologic motion of the distal tibiofibular joint, which may adversely affect ankle biomechanics, as reflected in a smaller joint contact area and a decrease in anterior and posterior drawer tests.⁶⁵ Clinically, outcomes one year after syndesmotic screw fixation were significantly better in patients with removed, fractured, or loosened screws, according to a retrospective analysis.⁶⁶

Tricortical screws may allow for more physiologic motion, with a resultant higher rate of associated hardware loosening.⁶⁷ Bioabsorbable screws as an alternative to metallic screws have comparable biomechanical properties and outcomes, while obviating the need for removal.⁶⁸ Similar outcomes exist for all fixation constructs regarding pain, range of motion, and functional assessments.

Suture-button devices are still a relatively new technology. Klitzman et al. conducted a cadaveric study to analyze suture-button biomechanics in comparison to screw fixation. They found that suture-button fixation preserved reduction after cycling with submaximal loads to a degree similar to an intact syndesmosis. Also, increased physiologic movement of the fibula in the sagittal plane was noted when compared to tricortical screw fixation.⁶⁹ This must be tempered with Teramoto et al., who found a potential limitation of suture-button devices to be insufficient fixation in multidirectional testing.⁷⁰

One study found screw fixation to be closer to native ankle kinematic motion in anterior/posterior and medial/lateral motions, while suture-buttons were closer to native fibular rotation. They also concluded that FiberWire-buttons were consistently unable to maintain reduction with the forces applied, while suture endobuttons had a performance equal to or greater than screw fixation when subjected to rotational torque.⁷¹

There are other construct types and concepts that have been reported. Kirschner wire fixation has been shown to be comparable to screw fixation in Weber C fractures with regard to joint motion and contact surface.⁶² This technique may provide sufficient stabilization in select fractures with syndesmotic injury.⁷² Posterior malleolus fracture fixation re-establishes the posterior tibiofibular ligament of the syndesmosis, while potentially allowing for normal ankle kinematics. Posterior malleolus fixation alone has been shown to result in better clinical outcomes than when combined with transsyndesmotic screw placement.⁷³

With regard to clinical outcomes, no major differences have been noted in functional outcomes between single and double screws, tricortical and quadricortical screws, transsyndesmotic and suprasyndesmotic screws, stainless steel and titanium, or metal and bioabsorbable screws to date.^{72, 74-76} At present, there is no “ideal construct” for syndesmotic fixation and as such it should be selected based on surgeon preference and experience. Screw fixation remains the most stable and reliable construct in moderate to severe ankle injuries involving the syndesmosis.

Conclusion

Depending on the fracture type, different fixation constructs may be optimal. Orthopaedic surgeons should carefully select the fracture fixation construct in order to optimize stability and promote healing.