Biomechanical studies of an anterior pelvic external fixation frame intended for control of vertical shear fractures

Evaluating the stability of external fixators following pelvic injury: A systematic review of biomechanical testing methods

INTRODUCTION

This systematic review aims to identify previously used techniques in biomechanics to assess pelvic instability following pelvic injury, focusing on external fixation constructs.

METHODS

A systematic literature search was conducted to include biomechanical studies and to exclude clinical trials.

RESULTS

Of an initial 4666 studies found, 38 met the inclusion criteria. 84% of the included studies were retrieved from PubMed, Scopus, and Web of Science. The studies analysed 106 postmortem specimens, 154 synthetic bones, and 103 computational models. Most specimens were male (97% synthetic, 70% postmortem specimens). Both the type of injury and the classification system employed varied across studies. About 82% of the injuries assessed were of type C. Two different fixators were tested for FFPII and type A injury, five for type B injury, and fifteen for type C injury. Large variability was observed for external fixation constructs concerning device type and configuration, pin size, and geometry. Biomechanical studies deployed various methods to assess injury displacement, deformation, stiffness, and motion. Thereby, loading protocols differed and inconsistent definitions of failure were determined. Measurement techniques applied in biomechanical test setups included strain gauges, force transducers, and motion tracking techniques.

DISCUSSION AND CONCLUSION

An ideal fixation method should be safe, stable, non-obstructive, and have low complication rates. Although biomechanical testing should ensure that the load applied during testing is representative of a physiological load, a high degree of variability was found in the current literature in both the loading and measurement equipment. The lack of a standardised test design for fixation constructs in pelvic injuries across the studies challenges comparisons between them. When interpreting the results of biomechanical studies, it seems crucial to consider the limitations in cross-study comparability, with implications on their applicability to the clinical setting.

Biomechanical aspects of pelvic ring reconstruction techniques: Evidence today

Despite the remarkable advances achieved within the boundaries of the new discipline of Pelvic surgery, pelvic ring disruptions remain challenging and complex problems in orthopaedics. The long-term complications related to reconstruction techniques of these injuries have motivated researchers and surgeons to explore various alternative treatment modalities. Several biomechanical studies have addressed these issues. We performed a medline search including studies published during the last 30 years. Our search yielded 114 studies, 39 of which met the pre-specified inclusion criteria and were further critically analysed and discussed regarding the biomechanical aspects of pelvic ring reconstruction techniques. Based on observational approach and evaluation of the studies specific keypoints are highlighted comprising the clinical translation of the biomechanical supported findings.

Biomechanics of pelvic fixation

This article reviews a series of biomechanical studies performed in the author's laboratory, evaluating the instabilities produced by the more common and problematic pelvic ring fracture patterns and the increasingly popular and newly developed modes of internal fixation. Topics that are discussed include the following: anteroposterior compression injuries and the disrupted sacroiliac joint; sacral fractures; anteroposterior compression injuries and symphyseal fixation; new symphyseal plate designs; unstable pubic rami fractures; unstable fractures of the iliac wing; and resuscitation fixator biomechanics.

Clinical and biomechanical aspects of external fixation of the pelvis

OBJECTIVE: The aim was to evaluate the mechanical stability of several traditional and modern external fixators in unstable pelvic ring disruption. DESIGN: In a laboratory study external and internal fixation techniques were tested in seven fresh and five embalmed human pelves with a disruption of the pubic symphysis and one sacroiliac joint (type C1.2 injury according to the Tile-AO classification). BACKGROUND: Stability provided by external fixation depends upon many factors, with the residual pelvic stability being the most important. METHODS: Simulating a single-leg stance, the load was applied quasi-statically to the acetabulum of the unstable hemipelvis. Device failure was defined as displacement >10 mm either at the symphysis pubis or the sacroiliac joint. RESULTS: The frame with the highest failure load (fresh versus embalmed specimens) was the Egbers configuration with the AO fixator (analysis of variance; P < 0.05). Failure was noted at 114.9 N versus 129.5 N. Augmentation of the Mono-Tube by additional internal posterior osteosynthesis gave the following results: sacral bars 325.4 N versus 217.8 N, plate fixation 294.3 N versus 215.8 N, lag screws 338.4 N versus 215.8 N. Failure loads of hybrid fixation of the Orthofix were as follows: sacral bars 257.9 N versus 213.9 N, plate fixation 333.5 N versus 245.3 N, lag screws 397.3 N versus 280.6 N. The differences between the two fixators were not statistically significant. CONCLUSIONS: No single external frame provided sufficient stability. The addition of a posterior internal fixation significantly increased failure loads and controlled the weight-bearing pelvic elements.

Anterior and posterior internal fixation of vertical shear fractures of the pelvis

A laboratory study was undertaken to evaluate the effectiveness of alternative methods of fixation of unilateral vertical shear fractures of the pelvis. Prior to experimental testing, a biomechanical analysis was performed to estimate the forces that displace the hemipelvis in the presence of two different patterns of injury: an interforaminal sacral fracture and a disruption of the sacroiliac joint. These lesions were then experimentally created in five unembalmed human pelvises and sequentially fixed with an external Hoffmann frame alone, a Hoffmann frame with sacral bars, or sacral bars with either one or two bone plates placed across the symphysis pubis. Each pelvis/fixator system was mechanically tested by loading along the direction of the resultant muscle force predicted by the biomechanical analysis. During loading the proximal migration of the ilium was continuously recorded with a transducer. At intervals during loading, the three-dimensional displacement of the detached hemipelvis at the pubic symphysis was also measured using a stereophotographic technique. With both the sacral fracture and the sacroiliac disruption, the addition of posterior sacral rods substantially increased the strength and rigidity of fixation provided by the Hoffmann fixation frame alone (p less than 0.01). In specimens with a sacral fracture, the use of anterior plates with posterior rods restored 65-71% of the strength of the intact pelvis, in comparison with 46% observed with the combination of sacral rods with an anterior Hoffmann frame (p less than 0.01). All of the methods of fixation evaluated in this study were less successful in stabilizing the sacroiliac disruption.(ABSTRACT TRUNCATED AT 250 WORDS)