Comparative Strength of Three Methods of Fixation of Transverse Acetabular Fractures

Abstract

With the advent of percutaneously placed lag screws for fixation of acetabular fractures, this study evaluated the strength of lag screw fixation compared with traditional fixation techniques of transverse acetabular fractures. Ten formalin-treated human, cadaveric pelvic specimens with bilateral, transtectal transverse acetabular fractures were used for this study. The right acetabular fractures were fixed with a five-hole plate and four screws with the central hole spanning the posterior fracture site. The left acetabular fractures were fixed with two lag screws, one each in the anterior and posterior columns, or with a screw and wire construct stabilizing both columns. The specimens were loaded to implant failure. Stiffness, yield strength, maximum load at failure, and site of failure was recorded. The plate and screw construct showed significantly greater yield and maximum strength when compared with the two lag screws. The stiffness of the lag screw method was 39% higher than that of the plating method, but this result was not statistically significant. In addition, the plate and screw method provided significantly greater maximum strength than the screw and wire technique. The quadrilateral plate seemed to be the weakest area of fixation because 83% of the implant failures occurred in this region. In patients in whom the risks of formal open reduction and internal fixation of acetabular fractures outweigh the possible benefits, such as in patients with burns or degloved skin, the advent of computer-assisted and fluoroscopically guided percutaneous surgical techniques have been instrumental. This study showed there is greater strength of fixation with a plate and screw construct, possibly secondary to supplementary fixation distal to the quadrilateral plate. However, lag screw fixation provided relatively greater stiffness, which may account for its clinical success. Percutaneous lag screw fixation of appropriate transverse acetabular fractures is a viable option.

Displaced acetabular fractures routinely are treated surgically with formal open reduction and internal fixation. ^{9,10,12,17–19,22,23} However, traditional surgical treatment requires large incisions with extensive soft tissue dissection, which can be complicated by infection, wound problems, blood loss, abductor weakness, sciatic nerve palsy, and heterotopic ossification. ^{7,9,12,14,19,23,25} Nonsurgical treatment has been attempted to limit these potential complications.

Matta and colleagues ^17–20 advocated conservative treatment for displaced acetabular fractures that could be reduced within 2 to 3 mm. To achieve and maintain this reduction, the patients often were kept on prolonged bedrest in skeletal traction. This treatment modality also was associated with complications, including decubitus ulcers, pin tract infection, deep vein thrombosis, and respiratory compromise.

The complications associated with recumbency and with formal open reduction and internal fixation led to an increased interest in limited internal fixation. ^24,31,32 These techniques potentially could limit morbidity by providing adequate fixation to allow early mobilization of the patient and the hip without extensile incisions. In addition, percutaneous techniques are beneficial in patients with multisystem injuries, burns, and degloved skin (Morel-Lavellé lesion). ²⁴ Computed tomographic (CT) guided fixation of acetabular fractures with percutaneously placed lag screws has been used successfully. ² However, this technique is limited by fracture pattern, questionable sterility of the CT suite, radiation exposure, and prolonged surgical time. ² Parker and Copeland ²⁴ and Starr et al ³¹ described a method of percutaneous acetabular fracture fixation with cannulated screws and fluoroscopic guidance. This technique has expanded the fracture patterns amenable to lag screw fixation, including minimally displaced acetabular fractures with complex fracture patterns. More recently, a computer-assisted surgical technique for placement of lag screws across minimally displaced acetabular fractures was described. ^8,34 This technique not only potentially expands the fracture patterns that can be treated percutaneously but also limits radiation exposure.

With the increase in lag screw fixation of acetabular fractures, the current study was done to evaluate the stability of this fixation method compared with a more traditional fixation technique. To the authors’ knowledge, there is no previous study in which the fixation strength of lag screws alone in transverse acetabular fractures was evaluated. Only one study specifically addressed the biomechanics of transverse acetabular fracture fixation in a cadaveric model, but this study did not address the stability of lag screw fixation in both columns of the acetabulum. ²⁷ Another study used a synthetic hemipelvis model with simulated transverse acetabular fractures but also did not assess the stability of anterior and posterior lag screw fixation. ²⁹ In the current study, a two-lag screw construct for the fixation of transverse acetabular fractures is compared with that of standard plate and screw fixation. A third method, one anterior lag screw and supplementary posterolateral wire loop fixation, was evaluated concurrently.

MATERIALS AND METHODS

Ten formalin-treated human, cadaveric, pelvic specimens with their proximal femurs were used. There were six specimens from males and four specimens from females. After the soft tissues were removed, the specimens were divided into two groups with equal gender distribution and gross bone quality. A transverse, transtectal fracture of the acetabulum was created by corticotomy with a 0.4-mm saw blade (Hall-Zimmer, Warsaw, IN) from the medial portion of the weightbearing dome and directed toward the apex of the greater sciatic notch. An external rotation force was applied to the femur and pelvis to finish the corticotomy and disrupt the cancellous bone, thereby simulating a clinical fracture pattern.

Group I: Two-Screw Method

Each of the left acetabular fractures in Group I were fixed with two 6.5-mm, cancellous, lag screws measuring 90 mm in length (Fig 1A). The anterior column screw was passed from the lateral ilium, across the fracture line, to the pubic ramus ensuring that the threads of the screw were past the fracture line. The posterior column screw was placed from the outer table, across the fracture line at the quadrilateral plate, to the medial ischium. Likewise, the threads of the screw were passed distal to the fracture line.

Fig 1A–C.: Three methods of fixation of transverse acetabular fractures are shown: (A) Group I fractures fixed with the two-lag screw technique; (B) Group II fractures fixed with the screw and wire construct; and (C) all right acetabular fractures (Group III) fixed with the plate and screw method.

Group II: Screw and Wire Method

The left acetabular fractures in Group II were fixed with a screw and wire technique. A lag screw, identical to the anterior column screw used in Group I, was placed. In addition, a 1.2-mm figure of eight wire loop (Richards, Memphis, TN) was placed posteriorly and anchored on two, 3.5-mm bicortical screws posterolaterally (Fig 1B). The superior anchoring screw was placed from the posterior aspect of the acetabulum, superior to the transverse fracture line, heading anteriorly to anchor in the medial wall of the ilium. The inferior anchoring screw traversed the quadrilateral surface of the ischium. The wire loop passed over the middle portion of the fracture line and was perpendicular to it at the posterior surface of the pelvis.

Group III: Plate and Screw Method

The right acetabular fractures in both groups were fixed with posterior 3.5-mm five-hole pelvic reconstruction plates and four screws (Synthes, Paoli, PA). There were two screws above and below the fracture with an empty hole at the fracture line (Fig 1C). The medioinferior screw passed through the quadrilateral surface of the ischium.

Each hemipelvis was potted in a block of polymethylmethacrylate at the top of the iliac crest and at the ischial tuberosity. Two metal frames were placed on the testing table of the loading machine (LoCap, Tinius Olsen, Willow Grove, PA) (Fig 2), and the potted pelvis was placed on top of the metal frames with recessed rods securing the interface between the pelvis and the metal frames (Fig 3). The proximal femurs of the specimens were fixed to the load cell of the loading machine. Each hemipelvis was oriented so that the force into the acetabulum was directed 45° superomedially and 15° posteriorly in the sagittal plane, which approximates the position of the femoral head in the standing position. ^1,14 A three-point bending test was performed until there was failure of fixation as determined by the change in the stress-strain curve (maximal loading 30,000 lb-feet at a speed of ¾ inch/minute). Precalibrated, interchangeable strain gauge load cells built into the loading machine measured force output, and the DS-50/2 acquisition and machine control system (Tinius Olsen, Willow Grove, PA) measured displacement, which allowed yield and maximum load to be calculated from the microprocessor output of the loading machine. Stiffness values were obtained from the slope of the lead-deflection curve of each pelvis. To avoid the individual differences in bone quality, the ratios of left to right sides of the paired data were used for comparison. Overall, eight specimens had complete data. The paired t test was used for comparison between the left side and the right side within the same group, and the Student’s t test was used for comparison between the two-screw method and the screw and wire method. The sites of failure in each pelvis were examined and recorded for analysis.

Fig 2.: The universal loading machine (LoCap, Tinius Olsen, Willow Grove, PA) and microprocessor for data analysis used in this study are shown. The calibrated load cell is seen in the center of the loading machine.

Fig 3.: Schematic drawing of the testing apparatus under the materials testing machine. Force was applied across the fracture site in a superomedial and posterior direction in the sagittal plane. A linear force was applied until failure of fixation was achieved.

RESULTS

The values of stiffness, yield load, and maximum load at failure of each fixation device are shown in Table 1. The mean left to right ratios of each group are shown in Table 2.

TABLE 1: Mechanical Properties of Fixation

TABLE 2: Mean Left to Right Ratio of Mechanical Properties

In Group I versus Group III analysis, yield and maximum load were significantly higher (both p < 0.01) with the plating method (Group III) than the two-screw method (Group I). The stiffness of the two-screw method was 39% higher than that of the plating method but was not significantly different (p > 0.05). This indicated that the two-screw method probably resists the micromovement between the fracture interfaces at least as well as the plating method, provided that loading did not exceed the strength that produced failure of the fixation.

In Group II versus Group III comparisons, the screw and wire method (Group II) showed no significant difference from the plating method (Group III) for stiffness and yield load (both p > 0.05). However, the plating method showed significantly greater maximum load to failure (p < 0.05).

The two screw (Group I) and screw and wire (Group II) methods were compared with the Student’s t test by evaluating the differences of the left to right ratios between Group I and Group II. The screw and wire method had significantly greater yield load (p < 0.05). Stiffness and maximum load were not significantly different (p > 0.05).

The quadrilateral plate seemed to be the weakest area of fixation. Seven of the specimens had failure of fixation by screw loosening in this region. In addition, three of the eight fractures with the plating method had slight anterior opening of the transverse fracture line before load testing. There was no implant fracture or catastrophic loss of reduction (Table 3). Fifteen of 18 (83%) screws anchored at the quadrilateral plate were loose.

TABLE 3: Failure Sites in Bending Test

DISCUSSION

The treatment options for fixation of acetabular fractures are a matter of controversy. ^{4–6,11,13,16,25,26,33} The options range from nonoperative treatment for undisplaced fractures, with the incumbent adverse effects, ^17,20 to operative fixation, ranging from percutaneous to extensile approaches. Specifically for transverse acetabular fractures, nonoperative treatment for infratectal fractures that have a large roof arc intact in the acetabulum, which enables containment of the femoral head, has been espoused. ²⁹ Letournel and Judet ¹⁵ showed that anatomic reconstruction of the hip is essential to achieve good functional results and reported that 82% of patients with anatomic reduction obtained excellent or very good long-term results. If a transverse fracture line transects the weightbearing dome, as in a transtectal fracture, a substantial increase in instability results. ²⁹ Rigid fixation is preferred to allow early range of motion (ROM) and to decrease the risks of recumbency. ^{9,10,12,17–20,22,23} In addition, loss of reduction occurs at a rate of 3% to 5% in patients with fractures of the anterior and posterior columns. ^5,25 However, open reduction and internal fixation is associated with numerous potential complications, including infection, deep venous thromboses, nerve palsy, avascular necrosis of the femoral head, heterotopic ossification, blood loss, and soft tissue compromise. ^7,31 Mechanisms that have been implicated in the formation of heterotopic ossification include extensive stripping of gluteal musculature from the external iliac fossa and increased trauma to the abductor mechanism associated with extensile approaches. ³ Less invasive techniques have been sought to limit the morbidity of fixation of acetabular fractures. However, the stability achieved with these methods has not been biomechanically evaluated.

During postoperative ROM exercises, the acetabulum receives loads from the muscles crossing the hip. The adductors and abductors produce medial and cephalic forces. The quadriceps and hamstrings exert posterior forces across the hip. With the hip and the knee in flexion, the quadriceps muscle group produces posterior forces while the hamstring muscles generate posterior forces with the hip in flexion and the knee in extension. During weightbearing, the acetabulum also is subject to medial and cephalic forces. The cephalic loads do not produce shearing forces across a transverse acetabular fracture. However, the medially and posteriorly directed loads will produce a shearing force across a transverse acetabular fracture. Thus, it is thought that the medially and posteriorly directed loads are the primary causes of failure of fixation in transverse acetabular fractures, and any fixation construct should seek to counter these forces. ¹⁴

Mears and Rubash ²¹ reported that a transverse acetabular fracture stabilized with a posterior plate and an anterior lag screw is as rigid as anterior and posterior plating, and the previous construct is rigid enough to allow partial weightbearing. With stable fixation, early ROM could be initiated. The clinical results from the fractures stabilized with CT-guided, fluoroscopically assisted, and computer-aided surgical techniques suggest that percutaneous lag screw fixation provides a strong enough construct to allow early postoperative ROM. ^2,24,31,34

Biomechanical testing has attempted to address the stability of transverse acetabular fractures with more traditional fixation techniques. Sawaguchi et al ²⁷ repaired the anterior column with a plate or lag screw, and the posterior column was fixed with one of three different plates. No differences were reported between the various modalities. Simonian et al ³⁰ used a cadaveric model of T-type acetabular fractures. In their study, the fracture was repaired with an anterior plate, a posterior plate, or combined anterior and posterior plates. Their results showed no significant difference in the stability of the three modes of fixation. Shazar et al ²⁹ used a synthetic hemipelvis model to stabilize transverse acetabular fractures with an anterior column plate, a posterior column plate, an anterior plate with a posterior column screw, a posterior plate with an anterior column screw, two posterior plates, or an anterior and a posterior plate. The anterior column lag screw with the posterior column plate provided significantly stiffer fixation when anterior column displacement was assessed. When posterior column displacement was assessed, the anterior plate and the posterior lag screw provided significantly stiffer fixation.

Lag screw fixation has been advocated as an excellent method for stabilizing acetabular fractures because of their ability to provide interfragmentary compression. ^3,28,32 Stöckle et al ³² achieved anatomic reduction, with no more than 1 mm fracture displacement, in 40 of 51 acetabular fractures by the open placement of lag screws into the anterior or posterior column or both. Their inclusion criteria were displaced acetabular fractures with an articular stepoff or gap of more than 3 mm and a fragment size greater that 1 cm. At the 2-year followup, 38 of 44 patients had excellent or good clinical and radiologic results. Stöckle et al recommended using an additional reconstruction plate in patients with decreased compliance, osteopenic bone, or comminuted fracture fragments.

In the current experiment, the quadrilateral surface was the weakest site of fixation. Eighty-three percent of screw loosening occurred in this area. The authors suggest that screw fixation in this area should be avoided or supplemented. The plating method had two screws fixed in the distal fragment and thereby provided this supplemental fixation. The remaining two techniques relied on one distal screw, so the fixation failed when this one screw loosened. The superior maximum and yield strength of the plating technique can be attributed partially to this supplemental distal screw fixation and the added rigidity of the pelvic reconstruction plate. Schopfer et al ²⁸ had a 45% reduction in compliance values for posterior column fractures fixed with double versus single posterior column plating.

The two-screw method had higher stiffness than the other two fixation techniques, although the difference did not reach statistical significance. The contact of fracture interfaces and the elasticity of the fixation material will affect the stiffness strength in fracture fixation. The two-screw method provided even compression through the anterior and posterior fracture interfaces. The compression technique for plating is technically demanding. Imperfect prebending of the plate can produce a gap on the opposite side of the fracture interfaces and subsequently decrease the stiffness of fixation. This situation is clinically relevant when the anterior extension of the transverse fracture line is not directly visualized. In this study, three of eight fractures fixed with the plating technique had slight anterior opening, simulating the experience that can occur with posterior plating of transverse acetabular fractures during surgery. Stöckle et al ³² reported that plate osteosynthesis can lead to incongruities of the joint surface by fragment displacement because of eccentric loading while tightening the screws. Technical error may explain some of the relative greater stiffness of the two-screw method. More exact prebending may provide improved stiffness to the plate and screw method.

The screw and wire method can achieve even contact across the fracture interfaces, and thereby provide significant stiffness. However, this technique is limited by the tension on the wire loop and the elastic characteristics of the wire. The greater elasticity of the wire than that of the screw may explain the lower stiffness achieved with this method.

The current study is limited by the use of formalin-treated specimens. Fresh specimens or clinical trials would enhance subsequent studies. In addition, in the current study the forces placed on the fixation constructs are through the proximal femurs. Forces that more closely represent physiologic muscle loads may provide different results.

Open reduction and internal fixation with plate and screw constructs provide sufficient stability to allow early postoperative ROM. ^{9,10,12,17–19,22,23} Clinical experience suggests that percutaneously placed lag screws also provide sufficient stability for ROM with minimal morbidity. The current study confirmed the superior strength of plating; however, lag screw fixation may provide a stiffer construct. Percutaneously placed lag screws ^8,24,31,34 are another tool for the treatment of acetabular fractures.