Locking Cap Designs Improve Fatigue Properties of Polyaxial Screws in Upper Extremity Applications

Functionally graded 3D printed plates for rib fracture fixation

BACKGROUND

Design freedom offered by additive manufacturing allows for the implementation of functional gradients - where mechanical stiffness is decreased along the length of the implant. It is unclear if such changes will influence failure mechanisms in the context of rib fracture repair. We hypothesized that our novel functionally graded rib implants would be less stiff than controls and decrease occurrence of secondary fracture at implant ends.

METHODS

Five novel additively manufactured rib implants were tested along with a clinically used Control implant. Fracture reconstructions were modeled with custom synthetic rib bones with a transverse B1 fracture. Ribs were compressed in a cyclic two-point bend test for 360,000 cycles followed by a ramp to failure test. Differences in cyclic stiffness, 3D interfragmentary motions, ramp-to-failure stiffness, maximum load, and work to failure were determined.

FINDINGS

The Control group had lower construct stiffness (0.76 ± 0.28 N/mm), compared to all novel implant designs (means: 1.35-1.61 N/mm, p < 0.05) and rotated significantly more about the bending axis (2.7° ± 1.3°) than the additively manufactured groups (means between 1.2° - 1.6°, p < 0.05). All constructs failed via bone fracture at the most posterior screw hole. Experimental implants were stiffer than Controls, and there were few significant differences between functional gradient groups.

INTERPRETATION

Additively manufactured, functionally graded designs have the potential to change the form and function of trauma implants. Here, the impact of functional gradients was limited because implants had small cross-sectional areas.

Failure analysis of a locking compression plate with asymmetric holes and polyaxial screws

Locking compression plates (LCP) with asymmetrical holes and polyaxial screws are effective for treating mid-femoral fractures, but are prone to failure in cases of bone nonunion. To understand the failure mechanism of the LCP, this study assessed the material composition, microhardness, metallography, fractography and biomechanical performance of a retrieved LCP used for treating a bone fracture of AO type 32-A1. For the biomechanical assessment, a finite element surgical model implanted with the intact fixation-plate system was constructed to understand the stresses and structural stiffness on the construct. Also, to avoid positioning screws around the bone fracture, different working lengths of the plate (the distance between the two innermost screws) and screw inclinations (±5°, ±10° and ±15°) were investigated. The fracture site of the retrieved LCP was divided into a narrow side and broad side due to the asymmetrical distribution of holes on the plate. The results indicated that the chemical composition and microhardness of the LCP complied with ASTM standards. The fatigue failure was found to originate on the narrow side of the hole, while the broad side showed overloading characteristics of crack growth. When the screws were inserted away from the region of the bone fracture by increasing the working length, the stress of the fixation-plate system decreased. Regardless of the screw insertion angle, the maximum stress on the LCP always appeared on the narrow side, and there was little change in the structural stiffness. However, angling the screws at -10° resulted in the most even stress distribution on the fixation-plate system. In conclusion, the LCP assessed in this study failed by fatigue fracture due to bone nonunion and stress concentration. The narrow side of the LCP was vulnerable to failure and needs to be strengthened. When treating an AO type 32-A1 fracture using an LCP with asymmetrical holes and polyaxial screws, inserting the screws at -10° may reduce the risk of implant failure and positing screws around the fractured area of the bone should be avoided.

Suture Augmentation Neutralizes Deforming Muscular Forces in a Simulated 2-Part Osteoporotic Proximal Humeral Fracture Model

OBJECTIVES

To evaluate the contribution that tension-relieving sutures, placed between a proximal humeral locking plate and the rotator cuff muscles, had on preventing varus malalignment in an osteoporotic 2-part proximal humerus fracture model.

METHODS

A 2-part fracture model was created in 8 cadaveric specimens and then fixed with a lateral locking plate. A custom shoulder testing system was used to increase loading through the supraspinatus (SS) tendon to drive varus deformity. Trials were performed with no suture placement; SS only; SS and subscapularis (SB); and SS, SB, and infraspinatus. The primary outcome was contribution of each point of suture fixation to prevention of varus collapse.

RESULTS

Suture augmentation to the SS, SB, and infraspinatus significantly decreased humeral head varus collapse when compared with the plate alone at nearly all loads ( P < 0.05). There were no significant differences in humeral head varus collapse between the 3 suture constructs.

CONCLUSIONS

In our biomechanical evaluation of a simulated osteoporotic 2-part proximal humerus fracture with incompetent medial calcar, tension-relieving sutures placed between a lateral locked plate and the rotator cuff tendons prevented varus malalignment.

The addition of cerclage wiring does not improve proximal bicortical fixation of locking plates for Type C periprosthetic fractures in synthetic humeri

BACKGROUND

Treatment of proximal humerus periprosthetic fractures is challenging. It remains difficult to achieve robust fixation of the proximal fragment to the locking plate using cerclage wiring and/or unicortical screws. Use of polyaxial tangentially directed bicortical locking screws increases screw purchase, but it is unclear if this option provides robust fixation. This biomechanical study compares fixation of constructs using cerclage wires, bicortical locking screws, and a hybrid method utilizing both methods.

METHODS

Uncemented humeral stems were implanted into synthetic humeri and Type C periprosthetic fractures were simulated with a 1 cm transverse osteotomy. Distal ends of locking plates were secured with bicortical non-locking screws. The proximal ends were supported by either isolated cerclage wires, polyaxial locking screws, or a hybrid combination of both (n = 6 for each group). A universal test frame was used for non-destructive torsion and cyclic axial compression tests. 3-D motion tracking was employed to determine stiffnesses and relative interfragmentary motions.

FINDINGS

Isolated screw constructs showed significantly increased resistance against torsional movement, bending, and shear, (p < 0.05) in comparison to cerclage constructs. The hybrid construct provided no significant changes in stability over the isolated screw construct.

INTERPRETATION

Addition of cerclage wires in this synthetic bone model of Type C periprosthetic humerus fractures did not add significant stability to proximal bicortical locking plate fixation. Considering risks of tissue stripping and nerve injury, usage of cerclage wires in a similar clinical setting should be chosen carefully, especially when bicortical fixation around the prosthetic stem can be achieved.

One size may not fit all: patient-specific computational optimization of locking plates for improved proximal humerus fracture fixation

BACKGROUND

Optimal treatment options for proximal humerus fractures (PHFs) are still debated because of persisting high fixation failure rates experienced with locking plates. Optimization of the implants and development of patient-specific designs may help improve the primary fixation stability of PHFs and reduce the rate of mechanical failures. Optimizing the screw orientations in locking plates has shown promising results; however, the potential benefit of subject-specific designs has not been explored yet. The purpose of this study was to evaluate by means of finite element (FE) analyses whether subject-specific optimization of the screw orientations in a fixed-angle locking plate can reduce the predicted cutout failure risk in unstable 3-part fractures.

METHODS

FE models of 19 low-density proximal humeri were generated from high-resolution computed tomographic images using a previously developed and validated computational osteosynthesis framework. The specimens were virtually osteotomized to simulate unstable malreduced 3-part fractures and fixed with the PHILOS plates using 6 proximal locking screws. The average principal compressive strain in cylindrical bone regions around the screw tips-a biomechanically validated surrogate for the risk of cyclic screw cutout failure-was defined as the main outcome measure. The angles of the 6 proximal locking screws were optimized via parametric analysis for each humerus individually, resulting in subject-specific screw orientations (SSO). The average peri-implant strains of the SSO were statistically compared with the previously reported cohort-specific (CSO) and original PHILOS screw orientations (PSO) for females vs. males.

RESULTS

The optimized SSO significantly reduced the peri-screw bone strain vs. CSO (6.8% ± 4.0%, P = .006) and PSO (25.24% ± 7.93%, P < .001), indicating lower cutout risk for subject-specific configurations. The benefits of SSO vs. PSO were significantly higher for women than men.

CONCLUSION

The findings of this study suggest that subject-specific optimization of the locking screw orientations could lead to lower cutout risk and improved PHF fixation. These computer simulation results require biomechanical and clinical corroboration. Further studies are needed to evaluate whether the potential benefit in stability could justify the increased efforts related to implementation of individualized implants. Nevertheless, computational exploration of the biomechanical factors influencing the outcome of fracture fixations could help better understand the fixation failures and reduce their incidence.

A cadaveric comparison of two methods for isolated talonavicular arthrodesis: Two-screws versus plate with integrated compression screw

BACKGROUND

This study compared stiffness between two constructs for talonavicular arthrodesis: a dorsomedial plating system and two partially threaded cannulated cancellous screws. We hypothesized that the plate would exhibit greater stiffness and resistance to deformation during cyclic loading.

METHODS

The constructs were implanted in eight matched pairs of cadaveric feet and subjected to axial torsion, cantilever bending in two directions, and cyclic loading to failure.

RESULTS

The two-screw constructs were significantly stiffer in plantar-dorsal bending (p = .025) and trended towards a higher number of cycles before failure than the plate group (p = .087). No significant differences were observed in internal torsion (p = .620), external torsion (p = .165), or medial-lateral bending (p = .686).

CONCLUSIONS

This study provided the first biomechanical assessment of a plating system with an integrated compression screw, which was significantly less stiff than a two-screw construct when loaded from plantar to dorsal.

Calcar screw position in proximal humerus fracture fixation: Don't miss high!

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.