Introduction: Vertebral body osteophytes are common in elderly spines, but their mechanical function is unclear. Do they act primarily to reduce compressive stress on the vertebral body, or to stabilise the spine in bending?. Methods: Spines were obtained from cadavers aged 51–92yrs (mean 77yrs) with radiographic evidence of vertebral osteophytes (mostly antero-lateral). Twenty motion segments, from T5-T6 to L3-L4, were dissected and loaded a) in compression to 1.5kN, and b) in bending to 10–25Nm. Vertebral movements were tracked at 50Hz using an optical MacReflex system. Bending tests were performed in random order, in flexion, extension, and lateral bending. Resistance to bending and compression was measured before and after surgical excision of all osteophytes. Bone mineral content (BMC) of osteophytes was measured using DXA. ANOVA was used to detect changes after osteophyte excision, and regression was used to examine the influence of osteophyte size. Results: Compressive stiffness was reduced by an average 17% following osteophyte removal (p< 0.05). In flexion and extension, bending stiffness was reduced by 60% and 79% respectively (p< 0.01). In left and right lateral bending, stiffness was reduced by 42% and 49% respectively. Osteophyte removal increased the neutral zone and range of motion in each mode of bending, and changes were proportional to osteophyte mass and BMC (p< 0.01). Conclusion: Vertebral body osteophytes primarily stabilise the spine in bending, and do little to resist compression, despite their considerable BMC. Predictions of vertebral compressive strength based on BMC measurements are likely to be over-estimates if large osteophytes are present

Introduction. Luk (Luk et al. Spine vol 23(21) 2303-2307 1998) has shown that in posterior surgery, the correction achieved can be predicted by fulcrum bending films. The relevance to anterior correction has been disputed, as this commonly involves shortening the spine by the removal of intervertebral discs. The aim of the study was to see whether the pre-operative bending angle reflected the degree of correction achieved. Method. 91 patients with a structural thoracic curve had an anterior endoscopic correction using a single rod. The mean age was 16.1 years. (range 10-46) The majority of curves were Lenke type 1 (79%) or Type 2 (8%). In all cases disc clearance and bone grafting were performed. All had pre-operative fulcrum bending films. The mean Cobb angle achieved at the pre-operative bending film was compared with the post-operative correction at 2 months. The FBCI (Fulcrum Bending Correction Index) and correction rates were also calculated. The FBCI is calculated by dividing the correction rate by the fulcrum flexibility and expressed as a percentage. It takes into account the pre-operative flexibility of the curve. Results. The mean Cobb angle achieved at the pre-operative bending film was 20.4 degrees. The mean Cobb angle of the corrected curve at 2 months following surgery was 20.4 degrees, (p=0.96). The mean FBCI was 107%. The overall correction rate was 60.1%. Conclusion. In our series fulcrum bending films have been highly predictive of the correction achieved following anterior endoscopic correction. The correction rate of 60.1% is in keeping with other series. In addition, the FBCI was 107%. The instrumentation had corrected to the flexibility achieved at the time of the pre-operative bending films. This implies that the discectomies performed at time of surgery had not significantly increased the correction achieved

Introduction: Various plating devices and screw systems are available for single and multi-level cervical fusions. Recent reports regarding screw migration under torsional load and a “windshield wiper effect” has brought to light the importance of plate and screw design as well as the choice of graft. Aim: This study examined the relative stability of cervical plating systems under pure bending and axial-torsional fatigue using the Cloward type graft. Methods: Five fresh-frozen human cervical and 10 porcine spines assessed by dual-energy x-ray absorptiometry (DEXA) scanning and then reconstructed at the C. 2–3. and levels using the anterior Cloward technique. C. 4–5. Two different plating systems (a solid plate and a hollow plate) were used and alternated between the C2–3 and C4–5 levels. Strain gauges placed on the plates themselves. The systems were subjected to pure bending and torsional loading.. Five kilogram loads were used to apply bending moments to the spine and did not differ between the two systems evaluated. Bending moments and displacement angles were recorded for the pure bending loading regime and torque versus time was recorded for the torsional fatigue loading. Results: Strain gauge analysis revealed minimal strains on the plates under the loading conditions. Torque versus time was measured, and the decay constant was calculated from the decay curves. The hollow plating system decayed quicker than the solid plating system. Angular displacement under pure bending was minimal. The hollow system plate system resisted greater torque compared with the solid system. The decay curves eventually reached an asymptote for the both systems. This implied that the systems become stable under fatigue loading. The X-rays illustrated no failure at the screw/ bone interface (i.e. No “wiper” effect) after torsional fatigue

Introduction. Bending tests are commonly used to evaluate the mechanical behaviour of small animal bones. To test whole bones, it is normal that soft tissue should be removed before testing. However, cleaning the specimens might disturb the callus, interfering with the mechanical properties. This study compares mechanical properties of rat tibia between specimen with and without muscle cleaning. Materials and methods. 12 male Wistar rats aged 3–4 months were used. Soft tissues including skin and muscle were removed from right tibias (Group A), whereas muscles on the left tibia were left intact (Group B). 4-point bending was used to find the ultimate load, stress and Young's modulus. Results. All specimens displayed a basic failure pattern from load-displacement graph. Mean ultimate load of 105.2N (S.E.M.=7.60, n=12) for group A and 101.6N (S.E.M.=7.32, n=12) group B. Mean stress at failure of 281.50MPa (S.E.M.=34.98, n=12) was found for group A 288.70MPa (S.E.M.=20.83, n=12) for group B. Mean Young's modulus was 8.97GPa (S.E.M.=1.44, n=12) for group A and 10.05GPa (S.E.M=0.69, n=12) for group B. No statistical differences for any mechanical properties were found between groups for either t-test(p>0.05) or Bland-Altman plot. Discussion. No differences in mechanical properties were found with or without soft tissue. Therefore, biomechanical testing of small specimen bending without removing muscle can be used. The advantages of this technique are reduced specimen preparation time and decreasing risk of creating a stress raiser at the callus

Abstract

Introduction: Little is known about how the cervical spine resists the high complex loading to which it is often subjected in life. In this study, such loading was applied to cadaveric cervical motion segments in order to a) measure their strength in forward and backwards bending, b) indicate which structures resist bending most strongly, and c) indicate how compressive injury influences the bending properties.

Methods: Ten human cervical spines aged 65–88yrs were obtained post-mortem, dissected into 14 motion segments, and stored at −20°C. Subsequently, motion segments were defrosted and secured in dental plaster for testing on a hydraulic materials testing machine. An optical motion capture system recorded specimen movement simultaneously. Specimens were loaded in 2.5sec in combined bending and compression to reach their elastic limit in flexion, and then extension. Experiments were repeated following creep loading, removal of spinous processes, removal of apophyseal joints, and vertebral body compressive damage.

Results: On average, full flexion was reached at an angle of 7.2° and a bending moment of 6.8Nm. Full extension occurred at 9.2° and 9.0Nm. Creep loading reduced specimen height by 0.37mm, increased flexion by 1.5° (P< 0.01) but had little effect on extension. After creep, resistance to flexion came from the spinous processes and related ligaments (46%), apophyseal joints (30%), and disc (24%). Resistance to extension came from spinous processes (23%), apophyseal joints (45%), and disc (32%). The compressive strength of discvertebral body specimens was 1.87kN (STD 0.63kN). Compressive damage reduced specimen height by 0.83mm (STD 0.29mm). This reduced the disc’s resistance to flexion by 44% and extension by 18%.

Conclusion: Cervical motion segments have approximately 20% of the bending strength, and 45% of the compressive strength, of lumbar specimens of similar age. The relative weakness of the cervical spine in bending may influence the patterns of injury seen in “whiplash”.

Bi-cruciate stabilized (BCS) TKA is the prosthesis that aims to substitute bi-cruciate ligament with post-cam engagement. We estimated to describe the in vivo kinematics during deep knee bending in BCS and Cruciate retaining (CR) TKA with the same articular geometry. We analyzed 26 knees who agreed to the current investigation under institutional review board approval. 17 knees were implanted with BCS (Journey ∥BCS, Smith & Nephew. Memphis, US) and 9 knees with CR (Journey∥CR). Each patient was asked to perform deep knee bending under weight-bearing condition. To estimate the spatial position and orientation of the TKA, 2D/3D registration technique with single fluoroscopy was used. We evaluated anteroposterior (AP) translation of the nearest point from femoral component to tibial axial plane for medial and lateral sides, femoral external rotation relative to tibial component and post-cam engagement in BCS. Measurement results were analyzed using Wilcoxon test. Values of P<0.05 were considered statistically significant. Medial AP translation indicated 11.7±5.1% posterior movement in BCS and 4.0±6.6% anterior movement in CR from minimum flexion to 130°. Lateral AP translation indicated 28.9±11.4% posterior movement in BCS and 18.3±6.2% posterior movement in CR from minimum flexion to 130°. Femoral external rotation were observed in both group and the amount of rotation were 5.2°±4.5° in BCS and 8.2°±4.0° in CR. Anterior post-cam engagement was not observed in all cases (76.5%). But medial AP translation in BCS was anteriorly in shallow flexion angles compared to CR. It suggested that anterior post-cam engagement couldn't work in valid.

Background

Hip resurfacing has advantages for the young active patient with arthritis; maintaining a large range of motion, preserving bone stock, and reduced dislocation risk. However high serum metal ion levels with metal-on-metal resurfacing, and their clinical implications, has led to a decline in the use of hip resurfacing. Ceramic bearing surfaces display the lowest frictional torque and excellent wear rates. Recent developments have enabled large, strong ceramic materials to be used as resurfacing components. Any wear debris that is generated from these articulations is inert. However an all-ceramic hip resurfacing could be at risk of fracture at the head-stem junction. A new ceramic hip resurfacing system with a titanium-ceramic modular taper junction has been developed. The introduction of a taper introduces the potential for fretting corrosion; we sought to determine the extent of this in an in-vitro model, and compared this prosthesis to the conventional 12/14 titanium-cobalt chrome (Ti6Al4V-CoCr) taper junction.

Despite its clinical significance, metaphyseal fracture healing has received little attention in research and experimental models have been limited. In particular it is not known to what extent the mechanical environment plays a role in metaphyseal fracture healing. Recently, a new murine internal fixation plate has been developed to stabilise fractures in the distal femur under highly standardised conditions. Goal of the current study was to modify this design, in order to be able to evaluate the influence of the fixator bending stiffness on metaphyseal fracture healing in mice.

Adapting the existing single body design, resulting in low flexibility fixation, two new plates were developed with a decreased bending stiffness of approximately 65% and 45% of the original implant (100%). Pilot experiments were performed on 54 animals, whereas the mice were sacrificed and fracture healing assessed radiologically and biomechanically after 14 and 28 days.

MicroCT evaluation confirmed that the osteotomy was created in the trabecular, metaphyseal bone of the distal mouse femora. All bones showed progressive fracture healing over time, with decreased implant stiffness leading to increased periosteal callus formation.

These implants represent an important new research tool to study molecular and genetic aspects of metaphyseal fracture healing in mice under standardized mechanical conditions, in order to improve clinical treatment in challenging situations, such as in osteoporotic bone.

Introduction

According to previous reports, unilateral total knee arthroplasty (TKA) would produce the asymmetric changes of lower extremity in the coronal plane in patients with bilateral knee osteoarthritis (OA). To our knowledge, little attention has been paid to the alignment changes of trunk and contralateral limb. It was hypothesized that the unilateral correction of knee deformity would affect trunk bending in the coronal plane after unilateral total knee arthroplasty. The purpose of the current study was to investigate trunk bending in the coronal plane before and after the surgery.

Background

Trunk muscle activity and thoraco-lumbar kinematics have been shown to discriminate non-specific chronic low back pain (NSCLBP) subgroups from healthy controls. Thoracic spine kinematics and muscle activity whilst intuitively associated with NSCLBP, has received less attention and the possibility of intra-regional interactions remains an area for exploration.

Mechanical tests that have been carried out to validate finite-element models predicting vertebral strength concern vertebral bodies under axial compression. But in standing position gravity loads can induce a flexion component, especially for the last thoracic and first lumbar vertebrae. The aim of the study was to evaluate the strength of complete vertebrae under anterior compression.

15 isolated vertebrae T11-L2 (four women, one man, 88 ± 14 years old) were tested to failure. The load was applied at the one third of the vertebral body depth through a ball constrained in a hole. It was homogeneously distributed on the vertebral endplate through a polymetylmetacrylate (PMMA) layer which completely fills the concavity. The solid composed by the PMMA layer and the steel plate containing the hole for the ball was called “upper plate”. Its 3D orientation was assessed using the Polaris® motion capture system (accuracy: 0.6 mm, 0.6°) thanks to tripods. Before testing, the position of the marker-frames was assessed using 3D reconstructions (obtained by bi-planar X-rays) to express all the movements relatively to the vertebral frame.

The outcome data was the position of the upper plate. The load was calculated from the measurement of the vertical load (using the testing machine sensor) and the orientation of the upper plate (using the Polaris® system).

The mean flexion of the upper-plate is equal to 1° (± 0.7°) before the vertebra collapses. As this value is weak, the optoelectronic assessment could be removed during the test if the initial 3D orientation of the upper plate relatively to the vertebral frame is assessed.

This protocol allowed collecting with accuracy all the data necessary to validate models.

During certain motions, the disc is at risk of annular injury. Axial compression coupled with various combinations of excessive flexion, lateral bending or axial rotation has been shown to lead to disc injury. However, similar injuries have also been caused by repetitive activity at lower, more physiological ranges of motion. The primary objectives of this study were to determine the regions of largest shear strain experienced by disc tissues in six degrees of freedom (DOF), since shear is considered a likely tissue failure criterion, and to identify the physiological motions that may place the disc at greatest risk of injury.

A grid of wires was inserted into the mid-transverse plane of nine human lumbar discs that were subjected to each of six principal displacements and rotations. Stereo-radiographs were taken in each position and digitised for reconstruction of the 3D position of each grid intersection. Maximum shear strains (MSS) were calculated from relative grid-intersection displacements and normalised by the input displacement or rotation. Physiological MSS were calculated using the maximum reported physiological lumbar segmental motion for each DOF.

The largest MSS were found in the posterior, posterolateral and lateral regions of the disc. For the translation motions, lateral shear and compression produced the largest MSS (approx. 9%/mm). For the rotation motions, lateral bending had significantly larger MSS than all other tests (5.8±1.6 %/°, P< 0.001).

The physiological MSS was greatest for lateral bending, being significantly larger than all other motions (57.8±16.2%, P< 0.001). In addition, physiological MSS for flexion was also significantly larger than for all remaining motions (38.3±3.3%, P< 0.001).

This study has identified lateral bending and flexion as the lumbar segmental motions that may place the disc at greatest risk of injury. The exact failure criterion for intervertebral disc tissue is not known, and MSS was used because it is related to maximum and minimum principal strains, and it was shown that disc tears may be initiated by large interlamellar shear strains that dominate over radial and circumferential annular fibre strains. These results provide improved understanding of disc behaviours under loading and may also be of value validating finite element models.

Introduction

Recent studies on large diameter femoral head hip replacements have implicated the modular taper junction as one of the significant sources of wear and corrosion products and this has been attributed to increased torque and bending on the taper interface. The aim of this study was to assess the effect of frictional torque and bending moment on fretting corrosion at the taper junction and to investigate whether different material combinations also had an effect.

Study Design: Prospective Cohort Study.

Summary of Background Data: It has previously been suggested that fulcrum bending radiographs (Cheung et al Luk 1997) and traction radiography under anaesthetic (Davis et al 2003) predict the flexibility and correction obtained following surgery better than conventional supine bending radiographs.

Objective: To compare fulcrum bending radiographs and traction radiographs for the prediction of surgical correction of idiopathic scoliosis.

Subjects: The study was based on 16 patients with a diagnosis of idiopathic scoliosis who underwent corrective surgery.

Outcome measures: The Cobb angle of the major curve was compared on the standing AP and fulcrum bending radiograph taken in the pre-op assessment clinic, the traction film undertaken under anaesthetic immediately prior to surgery and the first post operative standing radiograph taken. The post operative correction of the major curve was analysed using regression techniques and adjusted for the base line curve angle of the major curve.

Results: The results were presented as an estimate of the parameter coefficient in the model associated with 95% confidence intervals. The median pre-operative Cobb angle of the major curve was 69 degrees, on the fulcrum bending film was 47 degrees, on the traction film was 30 degrees, and on the first post operative film was 30 degrees. There was no evidence to suggest that the fulcrum Cobb had an effect on the post operative correction of the major curve. There was however evidence to suggest that the traction Cobb angle had an effect on the post operative correction of the major curve (parameter estimate 0.87) 95% CI (0.174, 1.399), T value = 2.83, P = 0.016.

Conclusion: Traction radiographs under anaesthetic better predict the surgical correction obtained in adolescent idiopathic scoliosis compared to fulcrum bending radiographs. These two techniques have not been directly compared before.

Introduction

Gamma Irradiation is often considered the gold standard for sterilizing bone allograft. However, a dose dependant decrease in the static mechanical properties of gamma irradiated bone has been well established. Supercritical Fluid Sterilization (SCF) using carbon dioxide represents a potential alternate method to sterilize allografts. This study aimed to evaluate the effect of SCF on the static and dynamic (fatigue) properties of cortical bone in 3-point bending.

Introduction: Fractures in long bones are frequently managed with intramedullary implants, plates ore external fixators. X-ray images are normally used to determine the point of full weight bearing and implant removal. Plain radiographs give only poor information about the mechanical properties of the healing callus. Several quantitative Methods: like QCT and DEXA provide information about the density of the new bone, but the mechanical properties remain unknown. For direct monitoring of the mechanical properties of the healing callus a 4-point-stiffness device for small animals was constructed. This devise is used to detect the influence of degradable implants on bone healing. Long term aim is to develop “smart” implants that degrade during healing and speed up the healing process.

Materials and Methods: An uniplanar, bilateral external fixator was mounted on the tibiae of New Zealand White Rabbits after osteotomy and introduction of different degradable, intramedullar implants. The 4-point-bending measurement unit was temporarily fixed to record deflection with a non-contact displacement transducer. Load cells were instrumented to record the stepwise load increase (25g). The max. bending moment was only 0.14 Nm to avoid bending of the implant. Additional μ-CT analysis was conducted on the stiffness measurement days to quantify bone healing. After the in-vivo tests the stiffness measurement device was validated with ex-vivo measurements of bone models in a Material Test System (MTS).

Results: The bending stiffness unit showed a high precision with a standard deviation of 5.55E-04 N/μm and a mean deviation error of all models of 1.74E-04 N/μm. We found a significant non-linear correlation between the measured stiffness and the diameter of the models (p< 0.05, r2=0.96). Furthermore a significant correlation between the stiffness device and the MTS in vitro was shown (r2=0.96, p< 0.005). A significant correlation between the data of the bending stiffness device and the MTS was found for all animals (r2=0.64, p< 0.01). μ-CT analysis showed an increase in callus formation and density during the increase in bending stiffness.

Discussion: In this study a precise measurement unit to mirror the mechanical properties of healing bone is presented. The device was successfully tested in an in-vivo model of fracture healing. The healing of callus around different degradable implants can be monitored to develop implants that degrade during fracture healing to avoid stress shielding or implant removal. Not only data about the healing bone can be gatherd with the μ-CT analysis, but also processes around the implants can be well monitored to evaluate degradation and quality of the implants.

A multibody dynamics program (LifeMOD/KneeSIM, LifeModeler, Inc., San Clemente, CA) was used to simulate knee bending. A PFC Sigma® (DePuy, Warsaw, IN) rotating platform (RP) posterior cruciate retaining total knee was subjected to two cycles of knee bending up to 130 degrees of flexion. The RP model (Free RP) included experimentally determined torsional frictional behaviour for the insert-tray bearing as a function of axial load and rotational speed. The analysis was repeated with the exact same implant design, but with the insert locked (Fixed RP) to the tray to prevent internal-external (IE) rotation (a theoretical design). IE rotation and tangential traction (frictional) forces were calculated over the contact patches and averaged at the centres of pressure in the medial and lateral compartments.

Cross-wise tangential traction forces were greater for the Fixed RP than for the Free RP design in both medial and lateral compartments. The tangential traction forces arising from rolling and sliding may cause delamination of the polyethylene, especially if they act cross-wise to the main direction of motion of the contact patches, in accordance with the strain-softening effect proposed as a mechanism of wear for multi-directional motion. Even though the amount of cross-wise motion in existing total knee arthroplasty designs has been shown to be limited, the present study indicates that cross-wise traction forces are greater in a theoretical design which is restrained from rotation at the RP bearing. These theoretical results lend support to the notion that a rotating platform design may reduce wear by reducing cross-shear traction forces between the femoral component and the tibial insert.

Introduction

In recent years, there has been a growing interest, in many fields of medicine, in the use of bone adhesives that are biodegraded to non-toxic products and resorbed after fulfilling their function in contact with living tissue. Biomechanical properties of newly developed bone glue, such as adhesion to bone and elastic modulus were tested in our study.

The FBCI has been shown to be a better method for describing scoliosis correction because it takes spinal flexibility into consideration. 1

Objective: To use FBCI prospectively to compare the efficacy of four different posterior instrumentations in the correction of thoracic scoliosis.

Method: 123 idiopathic scoliosis patients with thoracic curves were surgically treated prospectively using 4 different posterior instrumentations: TSRH (n=35); ISOLA (n=33); CD-Horizon (CD-H: n=32); and Moss-Miami (MM: n=23). All the operations were performed by the same team of surgeons using standard techniques. The curve was measured using the Cobb’s method on the pre-operative PA standing, fulcrum bending and 1-week post-operative PA standing radiographs. The conventional correction rate and the FBCI were calculated. One-way ANOVA and independent sample t-test were used for statistical analysis.

Results: (1) There were no significant differences between any of the 4 instrumentations when assessed using the FBCI, however, the correction rate was better in CD-H than in ISOLA and TSRH (Table 1). (2) Higher FBCIs were observed in the stiff curve group (fulcrum flexibility £ 50%) compared with those in the flexible group (fulcrum flexibility > 50%), while the correction rates were lower in the former than in the latter (Table 2).

Discussion: Better correction rate obtained in the CD-H group was attributed to the more flexible curves rather than the instrumentation itself. In the flexible curve group, the instrumentations have been able to take up all the flexibility revealed by the fulcrum-bending radiograph. Although the correction rate was less in the stiff curve group, the FBCI showed that the deformity correction was actually more than that indicated by the fulcrum bending radiographs. One possible explanation of this phenomenon may be that the fulcrum-bending radiograph is less effective in eliciting all the flexibility in the stiff curves.

Conclusion: All 4 instrumentations were EQUALLY effective in correction of thoracic scoliosis when the curve flexibility was taken into consideration.