New method for detection of complex 3D fracture motion--verification of an optical motion analysis system for biomechanical studies

Hinge screw or no hinge stabilization provides decreased stability compared to hinge plate in a biomechanical evaluation of distal femoral derotational osteotomies

PURPOSE

Derotational distal femoral osteotomy (DFO) is the causal treatment for patients with femoral torsional deformity. The fixation is achieved by a unilateral angle-stable plate. Delayed- or non-unions are one of the main risks of the procedure. An additional contralateral fixation may benefit the outcome. Therefore, we hypothesize that primary stability in DFO can be improved by an additional fixation with a hinge screw or an internal plate.

METHODS

Derotational DFO was performed in 15 knees and fixed either with an angle-stable plate only (group 'None'), with an additional lateral screw (group 'Screw') or with an additional lateral plate (group 'Plate'). Biomechanical evaluation was carried out under axial loading of 150 N (partial weight bearing) and 800 N (full weight bearing), followed by internal and external rotation. After linear axial loading in step 1, a cyclic torsional load of 5 Nm was applied under constant axial load in step 2. In step 3, the specimens were unloaded. Micromovements between the distal and proximal parts of the osteotomy were recorded at each step for all specimens.

RESULTS

In step 1, the extent of micromovements was highest in group 'None' and lowest in group 'Plate' without being significantly different. In step 2, group 'Plate' showed significantly higher stability, reflected by less rotation and lower micromovements. Increasing the axial load from 150 to 800 N at step 2 resulted in increased stability in all groups but only reached significance in group 'None'.

CONCLUSION

An additional contralateral plate significantly increased stability in derotational DFO compared to the unilateral angle-stable plate only. Contrary, a contralateral hinge screw did not provide improved stability.

STUDY DESIGN

Experimental study.

LEVEL OF EVIDENCE

IV.

The Use of Optical Tracking to Characterize Fracture Gap Motions and Estimate Healing Potential in Comminuted Biomechanical Models of Surgical Repair

Fracture healing is stimulated by micromotion at the fracture site, whereby there exists an optimal amount of strain to promote secondary bone formation. Surgical plates used for fracture fixation are often evaluated for their biomechanical performance using benchtop studies, where success is based on overall construct stiffness and strength measures. Integration of fracture gap tracking to this assessment would provide crucial information about how plates support the various fragments present in comminuted fractures, to ensure there are appropriate levels of micromotion during early healing. The goal of this study was to configure an optical tracking system to quantify 3D interfragmentary motion to assess the stability (and corresponding healing potential) of comminuted fractures. An optical tracking system (OptiTrack, Natural Point Inc, Corvallis, OR) was mounted to a material testing machine (Instron 1567, Norwood, MA, USA), with an overall marker tracking accuracy of 0.05 mm. Marker clusters were constructed that could be affixed to individual bone fragments, and segment-fixed coordinate systems were developed. The interfragmentary motion was calculated by tracking the segments while under load and was resolved into compression-extraction and shear components. This technique was evaluated using two cadaveric distal tibia-fibula complexes with simulated intra-articular pilon fractures. Normal and shear strains were tracked during cyclic loading (for stiffness tests), and a wedge gap was also tracked to assess failure in an alternate clinically relevant mode. This technique will augment the utility of benchtop fracture studies by moving beyond total construct response and providing anatomically relevant data on interfragmentary motion, a valuable proxy for healing potential.

Biomechanical analysis of fixation methods in acetabular fractures: a systematic review of test setups

Purpose

Optimal anatomical reduction and stable fixation of acetabular fractures are important in avoiding secondary dislocation and osteoarthritis. Biomechanical studies of treatment options of acetabular fractures aim to evaluate the biomechanical properties of different fixation methods. As the setup of the biomechanical test can influence the experimental results, this review aimed to analyze the characteristics, comparability and clinical implications of studies on biomechanical test setups and finite element analyses in the fixation of acetabular fractures.

Methods

A systematic literature research was conducted according to the PRISMA guidelines, using the PubMed/MEDLINE and Web of Science databases. 44 studies conducting biomechanical analyses of fixation of acetabular fractures were identified, which met the predefined inclusion and exclusion criteria and which were published in English between 2000 and April 16, 2021. The studies were analyzed with respect to distinct parameters, including fracture type, material of pelvis model, investigated fixation construct, loading direction, loading protocol, maximum loading force, outcome parameter and measurement method.

Results

In summary, there was no standardized test setup within the studies on fixation constructs for acetabular fractures. It is therefore difficult to compare the studies directly, as they employ a variety of different test parameters. Furthermore, the clinical implications of the biomechanical studies should be scrutinized, since several test parameters were not based on observations of the human physiology.

Conclusion

The limited comparability and restricted clinical implications should be kept in mind when interpreting the results of biomechanical studies and when designing test setups to evaluate fixation methods for acetabular fractures.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00068-022-01936-9.

Prediction of interfragmentary movement in fracture fixation constructs using a combination of finite element modeling and rigid body assumptions

The amount of interfragmentary movement has been identified as a crucial factor for successful fracture healing. The aim of our study was to combine finite element analysis with a rigid body assumption to efficiently predict interfragmentary movement in fixed tibial fractures. The interfragmentary movement in a transverse tibial shaft fracture (AO/OTA type 42-A3) fixed with a locked plating construct was simulated using finite element analysis. In order to assess the contribution of the components on the resulting interfragmentary movement, the tibia, screws and embedding was either simulated deformable or as rigid body. The rigid and the deformable model accurately predicted the interfragmentary movement (R² = 0.99). The axial movement ranged between 0.1 mm and 1.3 mm and shear movements were between 0.2 mm and 0.5 mm. Differences between the two models were smaller than 73 μm (axial) and 46 μm (shear). The rigid body assumption reduced computation time and memory usage by up to 61% and 97%, respectively.

Is initial interfragmentary compression made to last? An ovine bone in vitro study

Interfragmentary compression, a major principle of fracture treatment, is clinically not quantified and might be lost quickly even without functional loads. We designed an experimental study hypothesizing that (1) compression can be controlled using either lag screw or compression plate, and expecting similar initial compression, (2) loss of interfragmentary compression through relaxation within one hour is reduced with neutralization locking plate next to lag screw compared to compression plate. Twelve ovine femora (N=6) and humeri (N=6) were assigned into groups: Group 1 received a 45° oblique osteotomy at mid-diaphysis and was fixated using a 3.5 mm interfragmentary lag screw and locking compression plate (3.5 mm LCP, DePuy Synthes) as neutralization plate. Group 2 received a transverse osteotomy and was fixated with dynamic compression using compression plate (LCP). Interfragmentary pressure and relative bone fragment displacements were recorded over one hour. Median loss of compression over one hour time (relaxation) were 0.52% in Group 1, and 0.17% in Group 2 (p>0.05). Median rotational displacements amounted to 0.46° for Group 1, and 0.31° for Group 2, and axial displacement to a median of -20 μm in Group 1 and 25 μm in Group 2. Ovine bone interfragmentary stress relaxation maintains compression over the first hour for lag screw with neutralization plate for an oblique fracture line or compression plate for a transverse fracture line. Measured compression forces around 100 N could be overcome by physiological tension loading in bending or torsion, necessitating for instance tension band plating, additional lag screws or absolutive stability.

Effect of bone quality and quantity on the primary stability of dental implants in a simulated bicortical placement

OBJECTIVES

Conventional dental implants inserted in the molar region of the maxilla will reach into the sinus maxillaris when alveolar ridge height is limited. When surgery is performed without prior augmentation of the sinus floor, primary stability of the implant is important for successful osseointegration. This study aimed at identifying the impact of bone quality and quantity at the implantation site on primary implant stability of a simulated bicortical placement.

MATERIALS AND METHODS

In our in vitro measurements, bone mineral density, total bone thickness and overall cortical bone thickness were assessed by micro-computed tomography (μCT) of pig scapulae, which resembled well the bicortical situation found in human patients. Dental implants were inserted, and micromotion between bone and implant was measured while loading the implant with an axial torque.

RESULTS

The main findings were that primary implant stability did not depend on total bone thickness but tended to increase with either increasing bone mineral density or overall cortical bone thickness.

CLINICAL RELEVANCE

Limited bone height in the maxilla is a major problem when planning dental implants. To overcome this problem, several approaches, e.g. external or internal sinus floor elevation, have been established. When planning the insertion of a dental implant an important aspect is the primary stability which can be expected. With other factors, the dimensions of the cortical bone might be relevant in this context. It would, therefore, be helpful to define the minimum thickness of cortical bone required to achieve sufficient primary stability, thus avoiding additional surgical intervention.

The effect of coronal splits on the structural stability of bi-condylar tibial plateau fractures: a biomechanical investigation

INTRODUCTION

Surgical treatment of bi-condylar tibial plateau fractures is still challenging due to the complexity of the fracture and the difficult surgical approach. Coronal fracture lines are associated with a high risk of fixation failure. However, previous biomechanical studies and fracture classifications have disregarded coronal fracture lines.

MATERIALS AND METHODS

This study aimed to develop a clinically relevant fracture model (Fracture C) and compare its mechanical behavior with the traditional Horwitz model (Fracture H). Twelve samples of fourth-generation tibia Sawbones were utilized to realize two fracture models with (Fracture C) or without (Fracture H) a coronal fracture line and both fixed with lateral locking plates. Loading of the tibial plateau was introduced through artificial femur condyles to cyclically load the fracture constructs until failure. Stiffness, fracture gap movements, failure loads as well as relative displacements and rotations of fracture fragments were measured.

RESULTS

The presence of a coronal fracture line reduced fracture construct stiffness by 43% (p = 0.013) and decreased the failure load by 38% from 593 ± 159 to 368 ± 63 N (p = 0.016). Largest displacements were observed at the medial aspect between the tibial plateau and the tibial shaft in the longitudinal direction. Again, the presence of the coronal fracture line reduced the stability of the fragments and created increased joint incongruities.

CONCLUSIONS

Coronal articular fracture lines substantially affect the mechanical response of tibia implant structures specifically on the medial side. With this in mind, utilizing a clinically relevant fracture model for biomechanical evaluations regarding bi-condylar tibial plateau fractures is strongly recommended.

Locking plate constructs benefit from interfragmentary lag screw fixation with decreased shear movements and more predictable fracture gap motion in simple fracture patterns

BACKGROUND

A mechanical characterisation of lag screw fixation plus locking plate - although clinically widely used as either "mixed fixation concept" or absolutely stable fixation - is so far missing. This study aimed to evaluate the influence of an interfragmentary lag screw on the resulting motion at the fracture site of locking plate constructs using a simple fracture at the distal femur.

METHODS

Human cadaver femora were in vitro loaded in torsion and axial bending-compression with and without lag screw fixation next to a locking plate fixation. In addition, two plate working lengths were tested. Interfragmentary movement was measured optically.

FINDINGS

Axial interfragmentary movement is reduced with lag screw (102 mm plate working length, 1000 N, mean): 0.28 mm versus 0.82 mm. With lag screw, the fracture gap stays closed with mean normal interfragmentary movement ≤0.03 mm. Fracture gap tends to open without lag screw: normal interfragmentary movement up to -0.29 mm. Reduction of shear interfragmentary movement was observed throughout all tested loads and groups. Mean true shear remains generally low with lag screw (≤0.42 mm) compared to without lag screw (≤1.46 mm). We also found that interfragmentary movement variance decreases with lag screw, especially for longer plate working length.

INTERPRETATION

An interfragmentary lag screw next to locking bridge plating reduces fragment motion in vitro for a simple fracture pattern and provides a sufficient tool to decrease detrimental shear movements. Prospective clinical trials with interfragmentary lag screw fixation should prove these findings in wide clinical use to treat simple fracture patterns.

Opening-wedge osteotomies of the distal femur: minor advantages for a biplanar compared to a uniplanar technique

PURPOSE

Valgus malalignment of the distal femur may be treated with corrective osteotomy. The purpose of this study was to compare the primary stability of a lateral opening-wedge osteotomy (LOWO) using a uniplanar compared to a biplanar technique. A study was carried out to test both surgeries, with both an intact medial cortex and with a deliberate attached cut of the medial cortex simulating a fracture. The primary hypothesis was that the biplanar technique provides higher axial and torsional stiffness. It was further hypothesized that the mechanical superiority of the biplanar technique would not be affected in the case of breakage of the far medial cortex.

METHODS

A LOWO was performed in ten synthetic femora (#3406 left large Femur, 4th Generation, Sawbones, Malmö, Sweden) using a lateral angle stable locking plate (NCB© Distal Femur Plate, Zimmer Biomet, Warsaw, USA). A uniplanar osteotomy was performed in five femora, and a biplanar osteotomy was performed in five femora. The femora were tested for axial and torsional loads using a servo-hydraulic testing machine (Instron 8874, Instron Structural Testing GmbH, High Wycombe, UK).

RESULTS

Axial stiffness decreased significantly (p = 0.001) in both groups (20% in the uniplanar group and 28 % in the biplanar group) by cutting the medial cortex. The type of osteotomy had no significant effect. A slightly lower but not statistically significant axial stiffness was seen in the biplanar group both for intact and broken medial cortices. Internal torsional stiffness dropped by more than 30% for the uniplanar group and almost 24% for the biplanar group when the cortex was cut (p < 0.001). No significant change concerning internal torsional stiffness was found between the two groups. External torsional stiffness decreased by 32% for the uniplanar group and 4% for the biplanar group after the cortical cut (p = 0.029). No significant change concerning external torsional stiffness was found between the groups, but the biplanar group showed a tendency towards higher values of external torsional stiffness.

CONCLUSIONS

The axial and torsional stiffness of the implant-bone construct were not significantly affected by the type of osteotomy performed. Biplanar osteotomy tended to increase external torsional stiffness. In cases of fracture of the medial cortex, biplanar osteotomy significantly reduced the external rotation at the osteotomy and showed a significantly increased external torsional stiffness.

Biomechanical properties following open wedge high tibial osteotomy: Plate fixator combined with dynamic locking screws versus standard locking screws

BACKGROUND

Open wedge high tibial osteotomy is widespread in treating osteoarthritis of the knee. Bone healing of the gap and the necessity of bone substitutes are under discussion. Increasing movement of the osteotomy gap can improve bone healing, while excessive movement should be avoided. It was hypothesised that the use of dynamic locking screws, compared to standard locking screws, will increase interfragmental motion while construct stability persists.

METHODS

In 20 tibia sawbones open wedge high tibial osteotomy was performed using standard locking screws or dynamic locking screws. An incremental cyclic (2 Hz) compression to termination protocol was applied using a material testing machine (MTS MiniBionix 858). Relative motion of the osteotomy and construct stability were measured using an optical tracking system (PONTOS 5M system). Levels of significance were set to 0.05.

FINDINGS

19 Sawbones were statistically evaluated. Interfragmental motion increased significantly with dynamic locking screws compared to standard locking screws (P < 0.001). Lateral hinge fractured after a mean of 29,489 (dynamic locking screws) vs. 48,111 (standard locking screws) load cycles at a median load level 3 (50-1120 N) in dynamic locking screws group and at a median load level 5 (50-1440 N) (P = 0.002) in standard locking screws group.

INTERPRETATION

Using dynamic locking screws in open wedge high tibial osteotomy increases interfragmental motion within the range of optimal bone healing. A decrease in construct stability has to be considered compared to standard locking screws.

Is cannulated-screw fixation an alternative to plate osteosynthesis in open book fractures? A biomechanical analysis

OBJECTIVES

The current biomechanical work compares the symphyseal and sacroiliac stability obtained with two systems of bone osteosynthesis. The two methods of fixation compared were the 6-hole suprapubic non-locked plate and pubic fixation with two cannulated screws, a novel technique that can be applied percutaneously in the clinical practice. The aim of this study was to examine the validity of the use of two-cannulated-screws osteosynthesis in order to minimize the secondary effects of open fixation, especially in patients in whom an open reduction is contraindicated.

MATERIALS AND METHODS

A biomechanical study was designed in 9 fresh, human pelvis specimens, simulating an AO B1.1 type injury, using both fixation systems sequentially in each specimen. In both parts of the test, the specimens were subjected to an axial load of 300N. Displacements and rotations between the different pelvic elements were studied by means of a discrete set of points. The absence of differences between the two systems has been set as the null hypothesis.

RESULTS

There were significant differences in favor of the cross-cannulated screws in most of the displacements measured at the pubic symphysis and sacroiliac joint.

CONCLUSIONS

Fixation of the AO B1.1 type fractures with cross cannulated screws restores the biomechanical behavior of the pubic symphysis, obtaining better stability than fixation with the 6-hole non-locked plate. To date, no comparative, biomechanical studies have been conducted with these two systems of osteosynthesis. This study demonstrates that cross-cannulated screws fixation of the pubic symphysis in AO B1.1 pelvic fractures should be considered as an alternative to the conventional plating system.

Percutaneous iliosacral fixation in external rotational pelvic fractures. A biomechanical analysis

INTRODUCTION

Although the gold standard in open book pelvic fractures remains the pubic symphysis (PS) plate fixation, the clinical outcomes are not satisfactory, despite the excellent anatomical reduction assessed radiologically. Some authors suggest that residual instability of the posterior pelvic elements may be responsible for the chronic pain and the early osteoarthritic changes in the sacroiliac joint (SIJ).

OBJECTIVE

To evaluate whether the isolated posterior fixation with one or two iliosacral screws (ISSs) is sufficient to provide adequate stability for the treatment of Burgess Young APC-II (YB APC-II) type of pelvic ring injuries.

METHODS

Biomechanical experimental study using 7 fresh human pelvises, where an YB APC-II pelvic injury was previously implemented. The isolated posterior fixation of the pelvic ring with 1 or 2 ISSs directed in the S1 vertebra body was analysed in each specimen following an axial load of 300N. The different displacement of the SIJ and of the PS were analysed in all three spatial axes, using the validated optical measurement system 3D PONTOS 5M. A multivariate version of Friedman test (non-parametric ANOVA for repeated measures) was performed.

RESULTS

The isolated fixation of the SIJ with 1 ISS did not show any differences with respect to the intact pelvis (p=0.851). Regarding the PS, both type of fixations (with 1 or 2 ISSs) confirmed an acceptable correction and adequate control of the PS even though with some differences compared to the intact pelvis (p=0.01). The presence of the second ISS found not to offer any significant additional benefit. The three-dimensional analysis of the behaviour of the pelvic elements, in these two different types of fixation, did not show any statistical significant differences (p=0.645).

CONCLUSION

The posterior fixation with ISS can represent an alternative option for treatment of pelvic injuries associated with rotational instability. Further prospective clinical studies are necessary to determine, the influence of the residual pubic symphysis mobility in the every day life, when the above-mentioned technique is applied.

Complex distal humerus fractures-comparison of polyaxial locking and nonlocking screw configurations-a preliminary biomechanical study

OBJECTIVE

The investigation hypothesized that in current anatomical precontoured plates, angular stability plays only a minor role for the efficacy of the osteosynthesis at the distal humerus.

METHODS

An AO C2.3 fracture model was simulated and osteosynthesis performed with plates positioned in parallel. System rigidity and median fatigue limit were analyzed in artificial bones and the cycles to failure in cadaver specimens. Loads were applied in anterior-posterior direction (75° flexion) and axial direction (5° flexion). Four composite bone groups were investigated as follows: (1) 2.7 mm polyaxial locking screws, (2) 3.5 mm polyaxial locking screws, (3) 3.5 mm polyaxial locking screws and a gap bridging screw, and (4) 2.7 mm nonlocking screws. Two cadaver groups were investigated with 3.5 mm diameter polyaxial locking (5) versus nonlocking screws (6).

RESULTS

There were no differences in stiffness found between the locking versus nonlocking constructs in artificial (1) versus (4) and in cadaver bones (5) versus (6). The larger screw diameter of 3.5 mm in combination with a gap bridging screw significantly increased construct stiffness by 25% (3). The median fatigue limit was significantly increased using larger screw diameters (2) and a gap bridging screw (3). In cadaver bones, the polyaxial locking screws constructs (5) resisted higher peak loads and more cycles until failure compared with nonlocking constructs (6).

CONCLUSIONS

System stiffness increases with larger screw diameters and becomes significant with additional gap bridging screws in artificial bones. The use of polyaxial locking screws in anatomical adapted plates becomes more important in poor bone quality.