Biomechanical stability of bioabsorbable screws for fixation of acetabular osteotomies

Advances in fixation strength of reorienting rectangular triple pelvic innominate osteotomies - A biomechanical investigation of two screw fixation techniques

BACKGROUND

Reorientating pelvic osteotomies are performed to prevent femoral-acetabular impingement or degenerative arthritis. A Toennis-Kalchschmidt triple pelvis innominate osteotomy is used in symptomatic patients. This study aimed to investigate the biomechanical behaviour of two different acetabular screw configurations for triple pelvis innominate osteotomy osteosynthesis.

METHODS

Two screw-orientation techniques in rectangular os ilium osteotomy were compared by osteotomising 12 artificial hemipelvises with triple pelvis innominate osteotomy protocol (fragment reorientation: 10.5° inclination and 10.0° anteversion) and randomising them in 2 groups (n = 6) for implantation with three 4.5 mm screws. Bidirectional group had a bidirectional screw orientation and Monoaxial group had a monoaxial direction of all three screws through iliac crest. All specimens were tested under progressively increasing cyclic loading until failure. Group-wise comparisons of acetabular cup medialisation, anteversion and inclination were evaluated via motion tracking at cycles 250, 500, 750, 1000, 1250, 1500, 1750, 2000, 2250, and 2500. Failure was defined as reaching 5° inclination or 5° anteversion.

FINDINGS

Acetabular cup medialisation (p ≤ 0.026), anteversion (p ≤ 0.021) and inclination (p ≤ 0.039) all increased significantly during testing in both groups. There were no significant differences for the group-wise comparisons at the cycle points defined in the methods (p ≥ 0.182). No significant differences were detected between groups for cycles to failure and failure load (p = 0.873).

INTERPRETATION

Bidirectional screw alignment does not lead to significant advantages compared to pure monoaxial if all three axial screws are evenly distributed over the osteotomy geometry. The triple pelvis innominate osteotomy is susceptible to changes in anteversion, inclination and medialisation under partial weight-bearing. Cautious rehabilitation protocols are recommended.

Percutaneous screw fixation of the pubic symphysis versus plate osteosynthesis: a biomechanical study

Objectives: To compare the stability of screw fixation with that of plate fixation for symphyseal injuries in a vertically unstable pelvic injury (AO/Tile 61-C1) associated with complete disruption of the sacroiliac joint and the pubic symphysis. Methods: Eight fourth-generation composite pelvis models with sacroiliac and pubic symphyseal disruption (Sawbones, Vashon Island, WA) underwent biomechanical testing simulating static single-leg stance. Four were fixed anteriorly with a symphyseal screw, and 4 with a symphyseal plate. All had single transsacral screw fixation posteriorly. Displacement and rotation were monitored at both sacroiliac joint and pubic symphysis. Results: There was no significant difference between the 2 groups for mean maximum force generated. There was no significant difference in net displacement at both sacroiliac joint and pubic symphysis. There was significantly less rotation but more displacement in the screw group in the Z-axis. The screw group showed increased stiffness compared with the plate group. Conclusions: This is the first biomechanical study to compare screw versus plate symphyseal fixation in a Tile C model. Our biomechanical model using anterior and posterior fixation demonstrates that symphyseal screws may be a viable alternative to classically described symphyseal plating.

Postoperative stability following a triple pelvic osteotomy is affected by implant configuration: a finite element analysis

BACKGROUND

The triple pelvic osteotomy is an established surgical method with multiple modifications regarding surgical technique and choice of implant. The stability of the osteotomy is affected by numerous factors, and among these, the three-dimensional implant configuration is a scientifically less explored aspect.

METHODS

We used a finite element model of a hemi-pelvis with a standardized triple osteotomy to calculate relative flexibility for loads in all translational degrees of freedom for five different implant configurations. Two of the configurations used entry points only feasible when implant removal was not necessary.

RESULTS

The stability of the osteotomy improved with an increased distance between the implants in the plane of the osteotomy as well as for a more perpendicular angle relative to the osteotomy plane. The implant configurations with more entry points available made this easier to adhere to.

CONCLUSION

The use of bioabsorbable implants may provide better opportunities for optimal implant constructs which can, to a certain degree, compensate for the lesser mechanical stiffness of bioabsorbable polymers as compared to metal implants.

Stability in Pelvic Triple Osteotomies in Children Using Resorbable PLGA Screws for Fixation

BACKGROUND

The triple pelvic osteotomy (TPO) is a major redirectional osteotomy used to improve the acetabular coverage of the femoral head in selected pediatric patients with hip disorders. Traditionally the iliac osteotomy is stabilized by metal screws that require a second surgery for removal. Despite favorable results for both adults and children in related pelvic osteotomies, resorbable implants have not previously been used for TPOs.This study aims to suggest a novel modified TPO surgical method in children using resorbable poly lactic-co-glycolic acid (PLGA) screws for fixation and to radiographically evaluate the postoperative stability achieved by these implants in a case series.

METHODS

All patients (n=11) with dysplasia of the hip (2), Perthe disease (5), or Down syndrome (4), who underwent primary TPO surgery during 2013 to 2019, using resorbable PLGA screws for fixation were included. The stability of the osteotomy was evaluated in the postoperative radiograph series using the acetabular index, migration percentage, Sharp's angle and the center-edge angle. The osteotomy angle was introduced as a parameter to confirm the postoperative integrity of the achieved correction. All cases were evaluated until radiographic healing of the iliac osteotomy. Hospital notes were analyzed for complications or local reactions in relation to the implants.

RESULTS

In all studied hips, the overall achieved correction was well maintained. The mean (SD) correction achieved, from the preoperative images to the last measurable postoperative image, was a 16.7 degrees decrease for Sharp angle, a 20.9 degrees decrease in acetabular index and a 24.7 degrees increase for the center-edge angle, respectively. There were no complications related to the bioabsorption of the implants.

CONCLUSION

The present findings suggest that the modified TPO method, using resorbable PLGA screws, provides sufficient stability and appears to be a promising alternative to traditional TPO. Avoiding implant removal is a major benefit in a pediatric population. Resorbable screws enable the surgeon to place implants with more degrees of freedom since later implant removal is not a limiting factor.

LEVEL OF EVIDENCE

Level IV-case series.

Comparing the mechanical strength of hip spica cast between a conventional and a new method of application

PURPOSE

The current technique of hip spica application is mostly based on a publication by Kumar (J Pediatr Orthop 1(1):97-99, 1981). We modified the technique of hip spica application in order to reduce the rate of breakage across the hip joint and designed this study to compare the strength between hip spica applied according to Kumar's technique and the new technique.

METHODS

We created 12 hip spica casts with 24 hips according to Kumar's technique, and another 12 casts according to a new technique. The two types of spica were tested with a mechanical testing machine (Instron 3365 series) by using compression loading to failure in flexion, extension, abduction and adduction. Data were analysed in SPSS version 20.0. Comparison of means was done with an independent T test for normally distributed data and the Mann-Whitney test for skewed data.

RESULTS

The new technique involved the creation of three slabs, instead of 14 slabs as described by Kumar. The loads to failure for hip spica cast applied according to the new technique were higher than those applied according to Kumar's technique, and the differences were statistically significant. The stiffness was also higher in spica casts applied with the new technique; the difference was not statistically significant under extension force.

CONCLUSION

Hip spica applied with the new technique was stronger than that applied according to Kumar's technique based on load to failure testing. The new technique of application would potentially reduce the risk of cast breakage during the management of developmental dysplasia of the hip (DDH) and femur fracture in children.

Biomechanical comparison of the fixation after curved periacetabular osteotomy using titanium and bioabsorbable screws

PURPOSE

We performed curved periacetabular osteotomy, a modification of the Bernese periacetabular osteotomy, to treat hip dysplasia. The purpose of our study was to compare the biomechanical stability of acetabular fragments fixed in place with 3 titanium screws, or with 2 or 3 bioabsorbable screws composed of particulate resorbable uncalcined hydroxyapatite (HA) and poly-L-lactic acid (PLLA) termed HA/PLLA screws.

METHODS

Curved periacetabular osteotomy was performed on 24 composite model hemipelves. Three groups of 8 specimens were affixed with 1) 3 titanium screws, 2) 2 HA/PLLA screws, or 3) 3 HA/PLLA screws. After fixation, the hemipelves were biomechanically attested with simulated push-off phase of the gait cycle using a servohydraulic material testing system.

RESULTS

The average failure loads were not significantly different among the3 groups tested. However, the group repaired with 3 HA/PLLA screws showed significantly greater stiffness than the other groups.

CONCLUSIONS

All 3 fixation methods provided sufficient stability to allow for early partial weight-bearing after surgery. The use of 3 HA/PLLA screws provided significantly greater stiffness for acetabular fragment fixation.

Biomechanical analysis of acetabular revision constructs: is pelvic discontinuity best treated with bicolumnar or traditional unicolumnar fixation?

Pelvic discontinuity in revision total hip arthroplasty presents problems with component fixation and union. A construct was proposed based on bicolumnar fixation for transverse acetabular fractures. Each of 3 reconstructions was performed on 6 composite hemipelvises: (1) a cup-cage construct, (2) a posterior column plate construct, and (3) a bicolumnar construct (no. 2 plus an antegrade 4.5-mm anterior column screw). Bone-cup interface motions were measured, whereas cyclical loads were applied in both walking and descending stair simulations. The bicolumnar construct provided the most stable construct. Descending stair mode yielded more significant differences between constructs. The bicolumnar construct provided improved component stability. Placing an antegrade anterior column screw through a posterior approach is a novel method of providing anterior column support in this setting.

The use of bioabsorbable screws to fix Type II odontoid fractures: a biomechanical study

OBJECT

Anterior screw fixation of the Type II odontoid fracture stabilizes the odontoid without restricting the motion of the cervical spine. The metal screw may limit bone remodeling because of stress shielding (if not placed properly) and limit imaging of the fracture. The use of bioabsorbable screws can overcome such shortcomings of the metal screws. The purpose of this study was to compare the strength of a 5-mm bioabsorbable screw with single 4-mm metal and double 3.5-mm lag screw fixation for Type II fractures of the odontoid process.

METHODS

Three different modalities of anterior screw fixation were used in 19 C-2 vertebrae. These fixation methods consisted of a single 5-mm cannulated bioabsorbable lag screw (Group A), a single 4-mm cannulated titanium lag screw (Group B), and two 3.5-mm cannulated titanium lag screws (Group C). Anteroposterior (AP) stiffness and rotational stiffness were evaluated in all constructs.

RESULTS

There was no statistical difference among the ages of the cadavers in each group (p = 0.52). The AP bending stiffness in Groups A, B, and C was 117 ± 86, 66 ± 43, and 305 ± 130 Nm/mm, respectively. The AP bending stiffness in Group C was significantly higher than that in Groups A and B (p = 0.01 and p = 0.001, respectively). The difference in AP bending stiffness values of bioabsorbable and 4-mm metal screws was not statistically significant (p = 0.23). The rotational stiffness of the double 3.5-mm metal screws was significantly greater than that of the 5-mm bioabsorbable and the 4-mm titanium screws.

CONCLUSIONS

Double screw fixation with 3.5-mm screws provides the stiffest construct in Type II odontoid fractures. Bioabsorbable lag screws (5 mm) have the same AP bending and rotational stiffness as the single titanium lag screw (4 mm) in odontoid fractures.

Locking plates have increased torsional stiffness compared to standard plates in a segmental defect model of clavicle fracture

OBJECTIVE

To evaluate the effect of locked plate technology to resist torsion in a clavicle fracture model of segmental bone loss.

METHODS

Forty-four synthetic clavicles were repaired with either 3.5 mm locked compression plate (LCP) or 3.5 mm low-contact dynamic compression plate (LCDCP). They were divided into two groups of 22 specimens. Each group was tested to evaluate torsional stiffness, load at failure, deflection at failure, and unconstrained plate motion.

RESULTS

LCP group showed significantly greater stiffness in torsion compared to the LCDCP group (p < 0.001). Average difference was 20.9%. Load at failure was not significantly different (p < 0.07). Deflection at failure was significantly less for the LCP group (p < 0.03). Unconstrained motion or plate 'looseness' was significantly less for the LCP group (p < 0.017).

CONCLUSIONS

In a simulated model of segmental clavicle fracture, a LCP provided more stiffness and less deflection than a low-contact dynamic compression plate.

Reconstruction plates for stabilization of mid-shaft clavicle fractures: differences between nonlocked and locked plates in two different positions

Reconstruction plates permit contouring to the irregular anatomic shape of the clavicle. This study evaluated the biomechanical stability of locking and nonlocking clavicle reconstruction plates for treating midshaft, transverse fractures, comparing anterior-inferior to superior plate position. Twenty-four synthetic clavicles with mid-shaft fractures were repaired with either a locking or nonlocking clavicle reconstruction plate in either the anterior-inferior or superior plate position (n = 6/group). Repaired constructs were tested in axial compression, axial torsion, and cantilever bending failure. In compression, anterior-inferior plates were significantly stiffer than superior plates and locked plates stiffer than nonlocked. In torsion, anterior-inferior plates were stiffer, with a significant interaction term that favored anterior-inferior locked and superior nonlocked plates. In cantilever bending, superior plates had a significantly higher bending failure load and stiffness. Anterior-inferior plates failed at a significantly lower load ( approximately 40 N or approximately 4 kg), which could potentially occur in the postoperative period.

Comparison of bioabsorbable versus metallic implant fixation for physeal and epiphyseal fractures of the distal tibia

BACKGROUND

Transepiphyseal screw fixation of displaced distal tibial epiphyseal fractures is the most common method of treatment for these intraarticular injuries. Recent literature indicates that retained transepiphyseal metallic screws cause an increase in ankle joint contact pressure, thus favoring screw removal. Our hypothesis is that bioabsorbable screw fixation is an alternative to metallic fixation, which offers similar results without the need for screw removal.

METHODS

This is a retrospective review of distal tibial epiphyseal ankle fractures treated with screw fixation. Two groups, those treated with bioabsorbable screw fixation (group B, n = 24) and those with metallic screw fixation (group M, n = 26), were compared (t test) for differences in clinical and radiographic outcomes.

RESULTS

Analysis of demographic data revealed no significant differences between groups for sex, ethnicity, age, and height. Group B was significantly heavier than group M (67.4 vs 55.6 kg; P = 0.0496). Each group had a similar number of Salter-Harris types III and IV medial malleolus fractures and transitional fracture types. There was no significant difference between groups in the time from injury to fixation or in operative time. Radiographically, there were no nonunions in either group, and at final follow-up, 1 patient in group B had distal tibial joint line irregularity versus 3 in group M. Clinically, there were no significant differences between groups in time to full weight bearing or time to full activities.There were fewer complications in group B. A single case of loss of reduction requiring revision fixation occurred in each group. There was one documented growth arrest in group M and 2 suspected growth arrests in each group. Two patients in group M were successfully treated for a superficial wound infection with oral antibiotics. Fourteen patients in group M underwent planned screw removal.

CONCLUSIONS

Bioabsorbable screw fixation can be used for distal tibial epiphyseal fractures with no increase in operative time, nonunion rate, number of unplanned secondary surgeries, or other complications. The use of bioabsorbable screws eliminates the need for epiphyseal screw removal. A prospective randomized study is planned to eliminate the potential selection bias and to standardize clinical and radiographic follow-up.

LEVEL OF EVIDENCE

Level III, case-control study.

Biomechanical evaluation of clavicle fracture plating techniques: does a locking plate provide improved stability?

OBJECTIVES

To evaluate the biomechanical properties of both plate location (superior versus anterior-inferior) and plate type Small Fragment Contourable Dual Compression Plate (CDCP) versus 3.5 mm Universal Locking System Contourable Dual Compression Plate (Locked CDCP) in a synthetic midshaft transverse clavicle fracture model.

METHODS

Twenty-four pre-osteomized synthetic clavicles were repaired with either CDCP or locked CDCP technology 3.5 mm plates in either the superior or anterior-inferior position to form 4 groups of 6 clavicles. These were subsequently tested to evaluate torsional and axial construct stiffness, as well as bending load to failure, bending failure stiffness, and method of failure.

RESULTS

In axial compression, locked CDCP constructs were significantly more stiff than CDCP constructs (p < 0.001), but no statistically significant effect of plate location was observed. Torsional tests demonstrated a significant 2-way interaction favoring locked CDCP plates in the superior position and standard CDCP plates in the anterior-inferior position (p < 0.001). Bending failure testing revealed that the superior plate location had higher load to failure and bending failure stiffness than the anterior-inferior location (p < 0.0001). In addition, the superior locked CDCP plates demonstrated significantly greater bending failure stiffness than superior CDCP plates (p < 0.0001).

CONCLUSIONS

Biomechanically, repairing a midshaft clavicle fracture with a superior plate was more favorable compared to anterior-inferior plating in terms of both load to failure and bending failure stiffness. Furthermore, superior locked CDCP plates show improved bending failure stiffness over superior CDCP plates.