Effect of Screw Perpendicularity on Compression in Scaphoid Waist Fractures

Finite Element Analysis of Postoperative Stability of Transverse Scaphoid Waist Fracture

Background and Purpose

Scaphoid waist fractures are often stabilised with compression screws, Kirschner wires (K-wires), or a combination of both. While clinical and bio-mechanical studies evaluating their utility are available, the ideal configuration of implant that would provide adequate stability to permit early use of the hand is debatable. We examined configurations of a single screw, one screw along with a K-wire, and two K-wires used for a transverse scaphoid waist fracture fixation aiming to assess the stability provided by each in the immediate postoperative period.

Methods

Computer-aided design (CAD) models of the scaphoid, K-wire, and headless compression screw were created. A transverse fracture was created at the scaphoid waist, and the CAD models of the screw and K-wire were used to fix the fracture in different configurations in a distal to proximal direction. Finite Element Analysis (FEA) was used to examine the strength of configurations when they were subjected to compression and distraction forces. The total maximum deformation (TDef) and factor of safety (FoS) for each configuration were calculated and used as indirect indicators of postoperative stability.

Results

When a single screw was used, the configurations with the screw directed posteriorly from either centre or anterior had the best combined TDef and FoS values. For one screw and one K-wire, the configuration with screw and K-wire parallel to each other with the screw located along the long axis in the AP projection and anterior to the K-wire in the lateral projection had the best combined TDef and FoS values. When using two K-wires, configurations with the two wires diverging proximally on the lateral projection had the best combined TDef and FoS values.

Conclusions

When fixing a transverse scaphoid waist fracture with a single screw, the screw directed posteriorly from either the centre or anterior aspect of the distal pole has the best stability, a parallel configuration has the best stability when fixing it using a screw and a K-wire, and divergent configuration has the best stability when fixing it with two K-wires only.

Excessive Derotational K-Wire Angulation Decreases Compression by Headless Compression Screw

Background  Scaphoid fracture is the most common carpal bone fracture. Open reduction internal fixation of scaphoid fractures typically undergo stabilization by a single headless compression screw (HCS). During screw insertion, a derotational Kirschner wire (K-wire) is often placed for rotational control of the near and far fragment. Questions/Purposes  The aim of this study was to determine if there is an angle of derotational K-wire placement in relation to the axis of a HCS that compromises the amount of compression generated at a fracture site by the HCS. We hypothesize that increased off-axis angle will lead to decreased compression across the fracture site. Methods  A Cellular Block 20 rigid polyurethane foam (Sawbones, Vashon, WA) scaphoid model was created to eliminate variability in bone mineral density in cadaveric bone. MiniAcutrak HCS screws (Acumed, Hillsboro, OR) were used for testing. Three conditions were tested: (1) HCS with derotational wire inserted parallel to the HCS (zero degrees off-axis); (2) HCS with derotational wire inserted 10 degrees off-axis; and (3) HCS with derotational wire inserted 20 degrees off-axis. Results  A statistically significant difference in the mean compression of the control group (56.9 N) was found between the mean compression with the derotational K-wire placed 20 degrees off-axis (15.2 N) ( p  = 0.001). Conclusions  Compression at the fracture site could be impeded by placing an excessively angulated off-axis derotation wire prior to insertion of the HCS. Clinical Relevance  Our study adds a new detail to the optimal technique of HCS placement in scaphoid fractures to improve compression and fracture union.

A customized percutaneous three-dimensional-printed guide for scaphoid fixation versus a freehand technique: a comparative study

The aim of this study was to compare the accuracy and reliability of percutaneous fixation of minimally displaced scaphoid fractures using a customized three-dimensional (3-D)-printed guide with a conventional freehand method. A prospective cohort of ten patients underwent scaphoid fixation with the aid of a customized 3-D-printed guide. The final screw position, total surgery time (minutes) and fluoroscopy time (seconds) was compared with a retrospective cohort of ten patients who underwent fixation with a conventional technique. There were no differences in final screw position between both methods. The patients in which the 3-D guide was used had a surgery time reduction of 43% and a fluoroscopy time reduction of 52% compared with the control freehand group. The use of a customized 3-D-printed guide permits a fixation that is as accurate as the standard freehand technique, with reduction in surgical time and intraoperative radiation exposure.Level of evidence: III.

Scaphoid fracture displacement is not correlated with the fracture angle

Classifications of scaphoid fractures associate the angle of the fracture with its stability. To examine this assumption, we measured acute scaphoid fracture angles and inclinations in relation to different scaphoid axes, using fracture displacement as an indicator of instability. We examined the effect of using different axes on the measurements of angles. CT scans of 133 scaphoid fractures were classified according to the location of the fractures. Using a three-dimensional computer model, we computed four scaphoid axes. For each fracture, we then measured the fracture angle and the direction of the fracture inclination in relation to each one of the axes. We found a correlation between displacement and the angles of proximal fractures using one of these axes (the surface principal component analysis axis). No such correlations were found for waist fractures, which were the majority of fractures. There were significant differences between the measurements made with different axes. The findings indicate that the angle of the fracture and the direction of the fracture inclination are minor factors in the displacement of most scaphoid fractures.Level of evidence: III.

Factors Associated with Scaphoid Nonunion following Early Open Reduction and Internal Fixation

Background  Nonunion after open reduction and internal fixation (ORIF) of scaphoid fractures is reported in 5 to 30% of cases; however, predictors of nonunion are not clearly defined. Objective  The purpose of this study is to determine fracture characteristics and surgical factors which may influence progression to nonunion after scaphoid fracture ORIF. Patients and Methods  We performed a retrospective case-control study of scaphoid fractures treated by early ORIF between 2003 and 2017. Inclusion criteria were surgical fixation within 6 months from date of injury and postoperative CT with minimum clinical follow-up of 6 months to evaluate healing. Forty-eight patients were included in this study. Nonunion cases were matched by age, sex, and fracture location to patients who progressed to fracture union in the 1:2 ratio. Results  This series of 48 patients matched 16 nonunion cases with 32 cases that progressed to union. Fracture location was proximal pole in 15% (7/48) and waist in 85% (41/48). Multivariate regression demonstrated that shorter length of time from injury to initial ORIF and smaller percent of proximal fracture fragment volume were significantly associated with scaphoid nonunion after ORIF (63 vs. 27 days and 34 vs. 40%, respectively). Receiver operating curve analysis revealed that fracture volume below 38% and time from injury to surgery greater than 31 days were associated with nonunion. Conclusion  Increased likelihood for nonunion was found when the fracture was treated greater than 31 days from injury and when fracture volume was less than 38% of the entire scaphoid. Level of Evidence  This is a Level III, therapeutic study.

Deviations in positioning variable pitch screws- scaphoid waist fractures

INTRODUCTION

Operative therapy using a headless cannulated variable pitch compression screw is the gold standard for the treatment of instable scaphoid fractures.

HYPOTHESIS

Deviation from the central placement is associated with a loss of stability and stiffness.

MATERIAL AND METHODS

An artificial bone model was manufactured and different screw positions (central, 10° and 20° to the long axis) were assessed. A shearing test with axial force on the 45° flexed scaphoid was applied.

RESULTS

The inserted variable pitch screw showed the highest stiffness and failure force in a position in the long axis. At 10 degrees, a slight decrease in stiffness (32.7N/mm±9.3N/mm) and failure force (41.6N±13.2N) was observed, while a significant reduction in stiffness (29.3N/mm±4.6N/mm) and failure force (50.3N±19.5N) was measured at 20 degrees.

DISCUSSION

Deviations in the angle of insertion of the compression screw cause loss in failure force, thus deviations from the central placement is associated with less stability and stiffness.

LEVEL OF PROOF

Controlled laboratory study (basic science study, biomechanical testing).

Antegrade Versus Retrograde Technique for Fixation of Scaphoid Waist Fractures: A Comparison of Screw Placement

Background  Scaphoid waist fractures are often treated using headless compression screws using dorsal or volar approaches. Objectives  The purpose of this study is to compare differences in screw position using a volar (retrograde) or dorsal (antegrade) approach. Patients and Methods  A total of 82 patients were retrospectively evaluated: 41 treated with a volar and 41 with a dorsal approach were selected. Postoperative radiographs were reviewed by three observers who rated screw location in the proximal pole, waist, and distal pole. Results  Thirty-four patients (83%) in the antegrade group had central screw placement in the waist of the scaphoid in posteroanterior and lateral planes compared with 14 (34.9%) in the retrograde group ( p  < 0.05). For the antegrade group, the screw was central in 217 of 246 zones (88.2%) compared with 127 of 246 (51.6%) in the retrograde group ( p  < 0.05). Conclusions  The dorsal antegrade approach appears to allow the surgeon to achieve central screw placement along all three scaphoid regions. Level of Evidence  This is Level III study.