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The retroacetabular angle determines the safe angle for screw placement in posterior acetabular fracture fixation.

ABSTRACT: Introduction. A method for the determination of safe angles for screws placed in the posterior acetabular wall based on preoperative computed tomography (CT) is described. It defines a retroacetabular angle and determines its variation in the population. Methods. The retroacetabular angle is the angle between the retroacetabular surface and the tangent to the posterior acetabular articular surface. Screws placed through the marginal posterior wall at an angle equal to the retroacetabular angle are extraarticular. Medial screws can be placed at larger angles whose difference from the retroacetabular angle is defined as the allowance angles. CT scans of all patients with acetabular fractures treated in our institute between September 2002 to July 2007 were used to measure the retroacetabular angle and tangent. Results. Two hundred thirty one patients were included. The average (range) age was 42 (15-74) years. The average (range) retroacetabular angle was 39 (30-47) degrees. The average (range) retroacetabular tangent was 36 (30-45)?mm. Conclusions. Placing the screws at an average (range) angle of 39 (33-47) degrees of anterior inclination with the retroacetabular surface makes them extraarticular. Angles for medial screws are larger. Safe angles can be calculated preoperatively with a computer program.

SUBMITTER: Tadros AM

PROVIDER: S-EPMC4045356 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature

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The retroacetabular angle determines the safe angle for screw placement in posterior acetabular fracture fixation.

Tadros Ayman M A AM Oxland Thomas R TR O'Brien Peter P

ISRN orthopedics 20130528

Introduction. A method for the determination of safe angles for screws placed in the posterior acetabular wall based on preoperative computed tomography (CT) is described. It defines a retroacetabular angle and determines its variation in the population. Methods. The retroacetabular angle is the angle between the retroacetabular surface and the tangent to the posterior acetabular articular surface. Screws placed through the marginal posterior wall at an angle equal to the retroacetabular angle a ...[more]

PMID: 24959359

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Project description:BACKGROUND:Geriatric acetabular fractures require fixation with sufficient primary stability to allow for immediate full-weight bearing. Minimally-invasive procedures would be desirable in order to keep perioperative morbidity low. The purpose of this study was to compare the biomechanical strength of lag screw-only fixation of anterior column posterior hemi-transverse (ACPHT) acetabular fractures to standard anatomical plate fixation. METHODS:Standardized ACPHT fractures were created in fourth generation synthetic pelvis models and stabilized by either an anatomical buttress plate (n?=?4) or by a screw-only construct (n?=?4). In a validated setup, a cyclic loading protocol was applied with increasing axial force (3200?cycles, 175?N to 2250?N). Construct survival, acetabular fracture motion, and mode of failure were assessed. RESULTS:The median number of cycles needed until failure of the construct occurred was 2304?cycles (range, 2020 to 2675) in the plate fixation group and 3200?cycles (range, 3101 to 3200) for the screw fixation constructs (p?=?.003). With regard to energy absorbed until failure, the plate fixation group resisted to 1.57?×?106?N*cycles (range, 1.21?×?106 to 2.14?×?106) and the screw fixation group to 3.17?×?106?N*cycles (range, 2.92?×?106 to 3.17?×?106; p?=?.001). All plate fixation specimens failed with a break-out of the posterior-column screw in the quadrilateral wing of the anatomical plate within a maximum load of 1750?N while the screw fixation constructs all survived loading of at least 2100?N. Acetabular fracture gap motion, acetabular rim angle, and medial femoral head subluxation as measures of fracture displacement were all not different between the two groups (p?>?0.1). CONCLUSIONS:In this in vitro biomechanical study, screw-only fixation of an ACPHT acetabular fracture resulted in at least as good construct survival as seen for standard buttress plate fixation. Both methods resisted sufficiently to forces that would be expected under physiologic conditions.

Project description:INTRODUCTION:In locked plate fixation of proximal humerus fractures, the calcar is an important anchor point for screws providing much-needed medial column support. Most locking plate implants utilize a fixed-trajectory locking screw to achieve this goal. Consequently, adjustments of plate location to account for patient-specific anatomy may result in a screw position outside of the calcar. To date, little is known about the consequences of "missing" the calcar during plate positioning. This study sought to characterize the biomechanics associated with proximal and distal placement of locking plates in a two-part fracture model. MATERIALS AND METHODS:This experiment was performed twice, first with elderly cadaveric specimens and again with osteoporotic sawbones. Two-part fractures were simulated and specimens were divided to represent proximal, neutral, and distal plate placements. Non-destructive torsional and axial compression tests were performed prior to an axial fatigue test and a ramp to failure. Torsional stiffness, axial stiffness, humeral head displacement and stiffness during fatigue testing, and ultimate load were compared between groups. RESULTS:Cadavers: Proximal implant placement led to trends of decreased mechanical properties, but there were no significant differences found between groups. Sawbones: Distal placement increased torsional stiffness in both directions (p?=?0.003, p?=?0.034) and axial stiffness (p?=?0.018) when compared to proximal placement. Distal placement also increased torsional stiffness in external rotation (p?=?0.020), increased axial stiffness (p?=?0.024), decreased humeral head displacement during fatigue testing, and increased stiffness during fatigue testing when compared to neutral placement. DISCUSSION:The distal and neutral groups had similar mechanical properties in many cadaveric comparisons while the proximal group trended towards decreased construct stiffness. RESULTS:from the Sawbones model were more definitive and provided further evidence that proximal calcar screw placements are undesirable and distal implant placement may provide improved construct stability. CONCLUSION:Successful proximal humerus fracture reconstruction is inherent upon anatomic fracture reduction coupled with medial column support. Results from this experiment suggest that missing the calcar proximally is deleterious to fixation strength, while it is safe, and perhaps even desirable, to aim slightly distal to the intended target.

Dataset Information

The retroacetabular angle determines the safe angle for screw placement in posterior acetabular fracture fixation.

Publications

The retroacetabular angle determines the safe angle for screw placement in posterior acetabular fracture fixation.

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