Isolated medial patellofemoral ligament reconstruction for patella instability is insufficient for higher degrees of internal femoral torsion.

PURPOSE: A medial patellofemoral ligament reconstruction (MPFL) with an additional derotational femoral osteotomy is suggested for patients suffering from patellar instability and an increased internal femoral torsion (IT). This biomechanical study investigated whether an isolated MPFL reconstruction could restore patellofemoral biomechanics for 10° and 20° relatively increased internal femoral torsion. METHODS: Eight fresh-frozen cadaver knees were tested on a specially designed knee simulator, which bend the knee from 0° to 90° flexion. Patellar motion (tilt and shift) and patellofemoral pressure (pressure shift, mean and peak pressure) were evaluated for 0°, 10° and 20° of IT with a native and reconstructed MPFL. RESULTS: An isolated MPFL reconstruction, compared to a native MPFL with the same femoral torsion showed a significant medial shift of the center of force (10° IT p < 0.001; 20° IT p = 0.02) and patella shift (10° and 20° IT p < 0.001) but no significant change in patella tilt (10° IT n.s.; 20° IT n.s.) for 10° and 20° IT. There was a significant medial shift in the center of force for 10° IT (10° IT p = 0.04) and a non-significant lateral shift for 20° IT (20° IT n.s.) in comparison to the native MPFL with 0° of femoral torsion. Patella shift was directed medially for 10° IT (10° IT p = 0.002). In knee flexion angles up to 30°, the patella remained more lateral for 20° IT and showed a different motion pattern (20° IT n.s.). Patella tilt showed a significant lateral tilt for 10° and 20° IT (10° IT p = 0.01; 20° IT p = 0.002). CONCLUSION: MPFL reconstruction as an isolated therapy only appears to be reasonable for 10° increased IT. While for an increased IT of 20°, a lateralizing force vector remains and an additional femoral derotational osteotomy is recommendable. These findings may assist surgeons in the decision making of surgical procedures in patients suffering from patella instability.

Biomechanical in vitro comparison of suture anchors for thumb UCL repair.

Different types of intraosseous suture anchors can be used for thumb ulnar collateral ligament (UCL) repair surgery. Some of them have already been tested biomechanically in intact bone; however, there exists little knowledge of their stability when implanted in avulsion fracture. In this biomechanical in vitro study, three anchor types (hard, soft and novel BoneWelding) were tested on 24 fresh frozen human thumbs from 12 body donors. After the repair of an iatrogenic ligamentous UCL rupture, the thumbs were cyclically loaded. A new set of anchors was implanted at the contralateral radial side of the same specimen into an iatrogenic avulsion fracture and pulled out of the bone. The most common mode of failure in ligamentous UCL repairs was the suture-ligament interface, especially when using soft anchors. In avulsion fractures, the novel anchor withstood significantly higher pullout forces than the hard or soft anchor (65 N vs. 42 N vs. 27 N; p = .006). The BW anchor provides sufficient anchorage in trabecular bone for UCL repair in case of avulsion fractures.

Ultrasound melted polymer sleeve for improved screw anchorage in trabecular bone--A novel screw augmentation technique.

BACKGROUND: Screw anchorage in osteoporotic bone is still limited and makes treatment of osteoporotic fractures challenging for surgeons. Conventional screws fail in poor bone quality due to loosening at the screw-bone interface. A new technology should help to improve this interface. In a novel constant amelioration process technique, a polymer sleeve is melted by ultrasound in the predrilled screw hole prior to screw insertion. The purpose of this study was to investigate in vitro the effect of the constant amelioration process platform technology on primary screw anchorage. METHODS: Fresh frozen femoral heads (n=6) and vertebrae (n=6) were used to measure the maximum screw insertion torque of reference and constant amelioration process augmented screws. Specimens were cut in cranio-caudal direction, and the screws (reference and constant amelioration process) were implanted in predrilled holes in the trabecular structure on both sides of the cross section. This allowed the pairwise comparison of insertion torque for constant amelioration process and reference screws (femoral heads n=18, vertebrae n=12). Prior to screw insertion, a micro-CT scan was made to ensure comparable bone quality at the screw placement location. FINDINGS: The mean insertion torque for the constant amelioration process augmented screws in both, the femoral heads (44.2 Ncm, SD 14.7) and the vertebral bodies (13.5 Ncm, SD 6.3) was significantly higher than for the reference screws of the femoral heads (31.7 Ncm, SD 9.6, p<0.001) and the vertebral bodies (7.1 Ncm, SD 4.5, p<0.001). INTERPRETATION: The interconnection of the melted polymer sleeve with the surrounding trabecular bone in the constant amelioration process technique resulted in a higher screw insertion torque and can improve screw anchorage in osteoporotic trabecular bone.

[Intervertebral cages from a biomechanical point of view]. / Intervertebrale Cages aus biomechanischer Sicht.

BACKGROUND: If lumbar interbody fusion is indicated, there are several options for instrumentation of the affected motion segment. Intervertebral cages are implanted in the disc to restore disc height and to stabilize the motion segment by tensioning the ligamentous structures. METHODS: Based on a selective literature search with the focus on biomechanical aspects of intervertebral cages, experimental and clinical studies are shown, interpreted, and discussed. RESULTS: In the literature, biomechanical flexibility tests of "stand alone" cages without supplemental instrumentation showed a limited stabilizing effect, particularly in extension and axial rotation, as well as an increased load transfer through the ventral column. Applying supplemental dorsal instrumentation can return the ventral/dorsal load sharing to the range of an intact motion segment and causes a marked increase of stability in all motion planes. Compared to bilateral dorsal instrumentation, unilateral dorsal instrumentation showed a reduced primary stability and leads to an asymmetrical loading of the cage which can cause unilateral loss of reduction. Nonmetallic cages with a stiffness adapted to bone allow better radiological evaluation of the bony fusion of the motion segment and theoretically have a reduced tendency to migrate. CONCLUSION: In combination with bilateral dorsal instrumentation, cage geometry and material have only a minor influence on primary stability and the main stability is provided by the internal fixator.

The influence of distal locking on the need for fibular plating in intramedullary nailing of distal metaphyseal tibiofibular fractures.

Using human cadaver specimens, we investigated the role of supplementary fibular plating in the treatment of distal tibial fractures using an intramedullary nail. Fibular plating is thought to improve stability in these situations, but has been reported to have increased soft-tissue complications and to impair union of the fracture. We proposed that multidirectional locking screws provide adequate stability, making additional fibular plating unnecessary. A distal tibiofibular osteotomy model performed on matched fresh-frozen lower limb specimens was stabilised with reamed nails using conventional biplanar distal locking (CDL) or multidirectional distal locking (MDL) options with and without fibular plating. Rotational stiffness was assessed under a constant axial force of 150 N and a superimposed torque of ± 5 Nm. Total movement, and neutral zone and fracture gap movement were analysed. In the CDL group, fibular plating improved stiffness at the tibial fracture site, albeit to a small degree (p = 0.013). In the MDL group additional fibular plating did not increase the stiffness. The MDL nail without fibular plating was significantly more stable than the CDL nail with an additional fibular plate (p = 0.008). These findings suggest that additional fibular plating does not improve stability if a multidirectional distal locking intramedullary nail is used, and is therefore unnecessary if not needed to aid reduction.

Clinical and biomechanical investigation of an increased articular cavity depth after distal radius fractures: effect on range of motion, osteoarthrosis and loading patterns.

INTRODUCTION: After fracture, distal radius malunion with dissociation of the volar and dorsal ulnar fracture fragments can lead to an increased articular cavity. PATIENTS AND METHODS: To investigate its clinical impact we retrospectively analyzed the outcome of 81 patients and simulated this form of malunion in a biomechanical experiment with six cadaver specimens in a dynamic loading set-up. RESULTS: In clinics, a higher arthritis stage was significantly correlated with an increased articular cavity depth and an increased anterioposterior distance. In cadaver specimens, a significantly decreased range of motion and significantly altered intraarticular contact characteristics were recognized for an increased cavity. CONCLUSION: Alterations in contact biomechanics could be one reason for the higher incidence of posttraumatic osteoarthritis when a deeper central impaction of the distal radius is present. From a clinical and experimental point of view, restoration of the normal shape of the distal radius is considered to minimize the risk for posttraumatic radiocarpal osteoarthritis.

[Biomechanical aspects of complex reconstructions following radical resection of thoracolumbar spinal tumors]. / Biomechanische Aspekte radikaler Resektionen und deren Rekonstruktion an der thorakolumbalen Wirbelsäule.

The total number of spinal tumors has increased over the past decade. However, the average survival time of tumor patients has increased due to improvements in the multidisciplinary treatment regimes. Therefore, radical tumor resection and complex reconstruction were developed in spinal surgery. Various reconstructive options for the throracolumbar spine are nowadays available and are depicted in this article. The success of complex reconstructive surgery relies on biomechanical principles and reconstruction is dependent on the size and location of the lesion, bone porosity and implant systems used. Special emphasis of this article focuses on en bloc vertebrectomy which is the most radical approach of spinal tumor surgery. The biomechanical aspects of different types of lesions and the reconstructive options are discussed in the context of the currently published literature.

Is augmentation a possible salvage procedure after lateral migration of the proximal femur nail antirotation?

INTRODUCTION: Failed fracture fixation of proximal femur fractures in the elderly usually results in prosthetic replacement. In case of the proximal femur nail antirotation (PFNA), during lateral blade migration or periimplant fracture at the femoral shaft, the femoral head remains intact and therefore a joint preserving procedure might be performed. However, implant anchorage in the femoral head after the second blade implantation generally results in reduced anchorage in the remaining cancellous bone. Therefore, we hypothesize that in the above mentioned cases augmentation of the PFNA blade could be a treatment option before prosthetic surgery has to be performed. MATERIALS: Biomechanical investigations were performed in eight fresh frozen femoral heads. Implant anchorage in case of blade extraction and reinsertion was investigated by rotation and pull out of a PFNA blade with a servohydraulic testing machine. After reinsertion of the blade and augmentation with bone-cement, the anchorage of the blade was investigated again to observe changes in torque and pull-out force. RESULTS: Rotational stability of the implant significantly increased after augmentation of the prior extracted PFNA blade. Pull-out strength was higher in the revised case than in the initial tests but without statistical significance. After augmentation, correlation between bone mineral density and pull-out strength which was found during initial pull-out disappeared. DISCUSSION: Augmentation of simulated blade exchange after lateral blade migration demonstrated a good anchorage. There was superior rotational stability in the revised case and no inferiority during pull out for the investigated specimens. Furthermore, augmentation could compensate for destroyed trabeculae and poor bone stock. It could furthermore be an option when a failed implant has to be replaced by a long PFNA in case of shaft fracture at the tip of the implant to increase anchorage in the femoral head. From a biomechanical point of view, reosteosynthesis and augmentation in selected cases of PFNA revision could be an alternative to initial prosthetic replacement. In the case of cut-out or medial implant protrusion, the suggested salvage technique is not feasible.

Biomechanical effect of bone cement augmentation on rotational stability and pull-out strength of the Proximal Femur Nail Antirotation™.

INTRODUCTION: After surgical treatment of osteoporotic hip fractures, complications such as implant cut-out are reported to be high and implant failure often is associated with poor bone quality. As augmentation is reported to enhance implant anchorage, the aim of our study was to investigate the effect of bone cement augmentation on the rotational stability and the pull-out resistance of the Proximal Femur Nail Antirotation™ (PFNa) blade. MATERIALS AND METHODS: A total of 18 fresh-frozen femoral heads (mean age 68 years, standard deviation (SD) 8.2) were scanned with quantitative computed tomography (qCT) for bone mineral density (BMD) measurements and instrumented with a PFNa blade. Nine specimens were augmented with a mean volume of 4.4 ml Traumacem V+. After cement consolidation, the blade was rotated for 60° for the rotational test. Subsequently, the blade was extracted from the specimens. Force, torque, displacement and angle were recorded constantly. RESULTS: In the rotational test, the mean maximum torque in the augmented group (17.2 Nm, SD 5.0) was significantly higher (p=0.017) than in the non-augmented group (11.7 Nm, SD 3.5). The pull-out test also yielded a significant difference (p=0.047) between the augmented (maximum pullout force: 2315.2N, SD 1060.6) and the non-augmented group (1180.4N, SD 1171.4). DISCUSSION: Augmentation of femoral heads yielded a significantly superior rotational stability, as well as an enhanced pull-out resistance, compared to the non-augmented state. However, the higher the BMD of the specimens, the lower was the effect of augmentation on the rotational stability. Therefore, augmentation can be a good clinical tool to enhance implant anchorage in osteoporotic bone.

Vertebroplasty with self-locking hexagonal metal implants shows comparable primary and secondary stiffness to PMMA cement augmentation techniques in a biomechanical vertebral compression fracture model.

With the growing incidence of vertebral compression fractures in elderly patients having a fair overall health condition, minimal-invasive treatment techniques are getting in focus of surgical therapy. Cement augmentation is widely performed and its complications and mechanical limitations are well described. Implants avoiding the side effects of cement augmentation while reaching the same level of stability would be desirable. The primary and secondary stability of a new augmentation method with self-locking hexagonal metal implants were investigated and compared with the performance of established augmentation options. 18 fresh-frozen human spinal specimens (Th12-L2/L3-L5) were tested with pure moments of 7.5 Nm in a six-degree-of-freedom spine simulator to investigate primary and secondary stability of three augmentation techniques: (1) vertebroplasty, (2) PMMA filled cavity and (3) hexagonal metal implants. An increasing three-step cyclic loading model was included. Elastic displacement and height loss under loading did not show significant differences between the three test groups. Investigation of primary and secondary stability evenly demonstrated comparable results for all techniques indicating an insufficiency to stabilise the fracture with higher load cycles. The newly introduced method for augmentation with the metal implant Spine Pearls achieved comparable results to bone cement based techniques in a biomechanical in vitro study. Midterm and longterm reduction preservation and ingrowth of the implants have to be proven in further studies.

Extent of corpectomy determines primary stability following isolated anterior reconstruction in a thoracolumbar fracture model.

BACKGROUND: Based on the development of minimal-invasive techniques and introduction of new implants enabling secure reconstruction an increasing number of patients are treated by isolated anterior column surgery. Most biomechanical studies dealing with thoracolumbar fracture models use worst-case scenarios of complete corpectomies to simulate vertebral body defects neglecting the influence of remaining cortical bone in partial corpus instability. Using a standardized partial and total corpectomy model we investigated the effect of the extent of corpectomy on stiffness in an anterior reconstruction model. METHODS: Twelve human thoracolumbar specimens (Th11-L3) were loaded in a spine simulator with pure moments in the three motion planes. Following intact testing partial corp- and discectomy and later complete corpectomy of L1 were performed. Defects were instrumented by vertebral body replacements and additional anterior plating systems bridging the defect from Th12 to L2. Intersegmental rotations were measured between Th12 and L2. FINDINGS: Significantly (P<0.05) increased range of motion was found in reconstructions of total compared to partial corpectomy. Total corpectomy reconstructions showed solely in lateral bending a significant reduction of range of motion compared to the intact state, while in axial rotation and flexion/extension it was significantly increased. Partial corpectomy reconstructions resulted in significantly reduced range of motion for lateral bending and flexion/extension compared to the intact specimen. INTERPRETATION: Isolated anterior reconstructions of the thoracolumbar spine revealed sufficient stiffness in the partial vertebral corpus defect. In contrast, total corpectomy did not show an adequate stiffness. Especially in regard to rotational stiffness additional posterior fixation has to be recommended.

The medial periosteal hinge, a key structure in fractures of the proximal humerus: a biomechanical cadaver study of its mechanical properties.

The medial periosteal hinge plays a key role in fractures of the head of the humerus, offering mechanical support during and after reduction and maintaining perfusion of the head by the vessels in the posteromedial periosteum. We have investigated the biomechanical properties of the medial periosteum in fractures of the proximal humerus using a standard model in 20 fresh-frozen cadaver specimens comparable in age, gender and bone mineral density. After creating the fracture, we displaced the humeral head medial or lateral to the shaft with controlled force until complete disruption of the posteromedial periosteum was recorded. As the quality of periosteum might be affected by age and bone quality, the results were correlated with the age and the local bone mineral density of the specimens measured with quantitative CT. Periosteal rupture started at a mean displacement of 2.96 mm (SD 2.92) with a mean load of 100.9 N (SD 47.1). The mean maximum load of 111.4 N (SD 42.5) was reached at a mean displacement of 4.9 mm (SD 4.2). The periosteum was completely ruptured at a mean displacement of 34.4 mm (SD 11.1). There was no significant difference in the mean distance to complete rupture for medial (mean 35.8 mm (SD 13.8)) or lateral (mean 33.0 mm (SD 8.2)) displacement (p = 0.589). The mean bone mineral density was 0.111 g/cm(3) (SD 0.035). A statistically significant but low correlation between bone mineral density and the maximum load uptake (r = 0.475, p = 0.034) was observed. This study showed that the posteromedial hinge is a mechanical structure capable of providing support for percutaneous reduction and stabilisation of a fracture by ligamentotaxis. Periosteal rupture started at a mean of about 3 mm and was completed by a mean displacement of just under 35 mm. The microvascular situation of the rupturing periosteum cannot be investigated with the current model.

A new distractable implant for vertebral body replacement: biomechanical testing of four implants for the thoracolumbar spine.

INTRODUCTION: Expandable titanium implants for vertebral body replacement in the thoracolumbar spine have been well established in the reconstruction of the anterior spinal column. Load transfer at the bone-implant interface remains a point of concern. The purpose of the study was to compare the performance in axial load transfer from the implant to the vertebral body in four different implants, all of them in clinical use to date. MATERIALS AND METHODS: We tested a second generation implant (Synex II) in comparison to three different expandable titanium cages: Synex I, Obelisc and X-Tenz. Twenty-four intact fresh frozen human lumbar vertebrae (L1-L4) were distributed into four identical groups according to bone mineral density (BMD). The BMD was determined by quantitative computed tomography (qCT). Specimens were loaded in craniocaudal direction with a material testing machine (Mini Bionix II) at a constant speed of 5 mm/min. Load displacement curves were continuously recorded for each specimen until failure (diminishment of compressive force (F) and/or obvious implant migration through the vertebral body end plate). One-way analysis of variance (ANOVA) and post-hoc tests (Bonferroni) were applied to detect differences at 1, 2, 3, and 4 mm displacement (F (1-4 mm)) between implant groups. RESULT: No significant differences were observed with regard to maximum compression force (F (max)) and displacement (d (max)) until failure: Synex II (1,782.3 N/4.67 mm); Synex I (1,645.3 N/4.72 mm); Obelisc (1,314.0 N/4.24 mm); X-Tenz (1470.3 N/6.92 mm). However, the mean compression force at 1-4 mm displacement (F (1-4 mm): 300-1,600 N) was highest for Synex II. The difference at 2 mm displacement was significant (p = 0.028) between Synex II (F (2 mm) = 879 N) and X-Tenz (F (2 mm) = 339 N). CONCLUSION: The modified end plate design of Synex II was found to perform comparably at least with regard to the compressive performance at the implant-bone interface. The risk of the new implant for collapse into the vertebral body might be reduced when compared to the competitors.

[Biomechanical study of a ventral stand-alone cage for the lumbar spine with and without additional posterior fixation]. / Biomechanische Studie über einen ventralen "stand alone cage" für die lumbale Wirbelsäule mit oder ohne zusätzliche dorsale Stabilisation.

AIM: Biomechanical study to examine the stability of a stand-alone cage (SAC; Topaz, Ulrich Medizintechnik, Ulm) incorporating integrated anterior fixation with and without additional posterior fixation. METHOD: Six human spinal specimens (L4/5) were loaded in a spine tester in the three main motion planes in the following states: (a) intact, (b) defect (nucleotomy), (c) SAC, (d) SAC+internal fixator (IF), and (e) SAC+translaminar facet screws (TFS). Facet joint translation (FJT) and range of motion (ROM) were measured and used to evaluate the stability of the tested states. RESULTS: The SAC stabilized the segment in comparison to the intact (a) and defect-containing (b) segments. The most rigid fixation was found for SAC+IF compared with the other states (a, b, c, e). The ROM and FJT of the SAC with the additional IF (d) showed a significant higher stability in all three motion planes. All differences in ROM and FJT between the tested states were statistically significant (p<0.05) except for the FJT SAC and SAC+TFS. CONCLUSION: The ventral Topaz SAC is a stable implant for the lumbar spine. Additional dorsal stabilization is an option to increase the stability.

En bloc spondylectomy reconstructions in a biomechanical in-vitro study.

Wide surgical margins make en bloc spondylectomy and stabilization a referred treatment for certain tumoral lesions. With a total resection of a vertebra, the removal of the segment's stabilizing structures is complete and the instrumentation guidelines derived from a thoracolumbar corpectomy may not apply. The influence of one or two adjacent segment instrumentation, adjunct anterior plate stabilization and vertebral body replacement (VBR) designs on post-implantational stability was investigated in an in-vitro en bloc spondylectomy model. Biomechanical in-vitro testing was performed in a six degrees of freedom spine simulator using six human thoracolumbar spinal specimens with an age at death of 64 (+/- 20) years. Following en bloc spondylectomy eight stabilization techniques were performed using long and short posterior instrumentation, two VBR systems [(1) an expandable titanium cage; (2) a connected long carbon fiber reinforced composite VBR pedicle screw system)] and an adjunct anterior plate. Test-sequences were loaded with pure moments (+/- 7.5 Nm) in the three planes of motion. Intersegmental motion was measured between Th12 and L2, using an ultrasound based analysis system. In flexion/extension, long posterior fixations showed significantly less range of motion (ROM) than the short posterior fixations. In axial rotation and extension, the ROM of short posterior fixation was equivalent or higher when compared to the intact state. There were only small, nonsignificant ROM differences between the long carbon fiber VBR and the expandable system. Antero-lateral plating stabilized short posterior fixations, but did not markedly effect long construct stability. Following thoracolumbar en bloc spondylectomy, it is the posterior fixation of more than one adjacent segment that determines stability. In contrast, short posterior fixation does not sufficiently restore stability, even with an antero-lateral plate. Expandable verses nonexpandable VBR system design does not markedly affect stability.

Three-dimensional stiffness in a thoracolumbar en-bloc spondylectomy model: a biomechanical in vitro study.

BACKGROUND: In selected cases, en-bloc spondylectomy is the only option to reach wide resection margins for patients with malignant tumours of the thoracolumbar spine. These patients must be also provided a secure initial stabilization of the spine and this is the role of vertebral body replacements employed with posterior fixation systems. The aim of this study was to determine the postimplantation stiffness of a connected vertebral body replacement pedicle screw system in different implantation scenarios following an en-bloc spondylectomy. Reconstruction was varied by posterior fixation lengths and axial compression forces during implantation. METHODS: Three-dimensional stiffness was assessed in 6 fresh frozen human spinal specimens (Th11-L3) using a six degree of freedom spine simulator. Following en-bloc spondylectomy reconstruction was performed using a carbon composite fibre vertebral body replacement connected to a posterior fixation system by two artificial pedicles. The spines were loaded with pure moments (7.5Nm) in the three main motion planes. The intersegmental rotations were measured between Th12 and L2. FINDINGS: Reconstructions using long posterior fixation modes demonstrated significant (P<0.05) higher stiffness compared to short posterior fixations in all motion planes. In axial rotation short posterior fixation modes failed to reach the values of the intact state. Neither high nor low axial compression force during implantation showed a significant impact on postfusional stiffness. INTERPRETATION: In this biomechanical model, the employed system should be implanted with a posterior fixation of two adjacent segments to the lesion in order to achieve a secure stabilization of the treated segment.

Biomechanical performance of the new BeadEx implant in the treatment of osteoporotic vertebral body compression fractures: restoration and maintenance of height and stability.

BACKGROUND: Vertebral compression fractures are counted among the most common complications of osteoporosis. For treatment, a new, alternative implant has been developed (BeadEx, Expandis, Hof HaCarmel, Israel). The aim of the present in vitro study was to evaluate whether this implant is able to restore the initial height and three-dimensional stability after fracture and whether it is able to maintain this height and stability during complex cyclic loading. METHODS: The BeadEx implant consists of small titanium rolls, which are pressed into the vertebral body through specially designed, hollow pedicle screws. The height and the three-dimensional flexibility of 18 bisegmental spine specimens (nine T12-L2, nine L3-L5) was measured, first, before and after creating a wedge compression fracture at the middle vertebral body (L1 resp. L4), second, after treatment of the fracture, and, third, during and after complex cyclic loading. The fractures were treated either with BeadEx plus internal fixator, BeadEx plus bone cement or vertebroplasty for comparison. FINDINGS: The height before fracture could almost be restored by BeadEx plus bone cement but not by BeadEx plus fixator and vertebroplasty. The total height loss after cyclic loading was smallest with BeadEx plus bone cement (in median -4.7mm with respect to the intact specimens) but -6.2mm with BeadEx plus fixator and -7.8mm with vertebroplasty. The three-dimensional stability of the specimens was clearly higher if treated with BeadEx plus fixator than with BeadEx plus bone cement or vertebroplasty. INTERPRETATION: From a biomechanical point of view, BeadEx plus bone cement can be recommended as an alternative to vertebroplasty in the treatment of osteoporotic vertebral body fractures. BeadEx plus fixator can be recommended if additional stability is needed.

Migration of two different cementless hip arthroplasty stems in combination with two different heads: a biomechanical in vitro study.

The aim of the present in vitro study was to evaluate migrational characteristics of cementless primary hip arthroplasty stems in combination with a diameter 50 mm head (hemiarthroplasty) and a diameter 28 mm head in and with a polyethylene cup (total hip arthroplasty) in fresh-frozen human specimens. Two different types (Endo SL, FMT) were implanted into seven pairs of fresh-frozen human femoral specimens. The implanted stems were combined with a diameter 50 mm head (hemiarthroplasty) on one side and with a polyethylene cup and a 28 mm head (total hip arthroplasty) on the other side. Dynamic mechanical loading was applied for 50,000 cycles while recording relative motions between stem and bone stock using a 3-D motion analysis system. The Endo SL stem showed a significantly higher amplitude of relative motion in all translational and rotational components regardless of the head used. In both stem types a strong tendency for higher axial migration with the diameter 50 mm head in comparison to the THA head was found. The lowest axial migration was found in the FMTstem in combination with the small head and a PE cup. The highest axial migration was found in the Endo SL stem combined with the diameter 50 mm head. Our results indicate that the head might play an important role for axial migration of cementless stems.

Cementless hemiarthroplasty in femoral neck fractures: evaluation of clinical results and measurement of migration by EBRA-FCA.

Aim of the present study was to evaluate migration rates of cementless primary hemiarthroplasty in acute femoral neck fractures. In a longitudinal, prospective study 46 patients were treated by cementless hemiarthroplasty. Clinical follow up was correlated with the EBRA-FCA method. In 30% of all patients stem migration amounted to more than 2 mm; further, these patients were seen to have a high level of activity. A high degree of migration in more than 30% of all patients requires critical scepticism toward further use of the investigated cementless stem as hemiarthroplasty. According to literature, migration of more than 2 mm suggests a high probability of early aseptic loosening. In patients with a low degree of activity good results could be observed; nevertheless, in patients with a high level of activity the combination of the investigated cementless stem with a solid fracture head cannot be recommended.

Influence of a dynamic stabilisation system on load bearing of a bridged disc: an in vitro study of intradiscal pressure.

In recent years, non-fusion implants to stabilise the lumbar spine have become more and more popular. However, little is known on the load bearing of such dynamic stabilisation systems. In order to investigate the load bearing of discs bridged with rigid and dynamic stabilisation systems, six lumbar cadaver spines were mounted in a spine tester and loaded with pure moments in the three main motion planes. Four different states of the specimens were studied: intact, destabilised, stabilisation with a Dynesys and stabilisation with an internal fixator. Intradiscal pressure (IDP) measurements were used to assess the load bearing of the bridged disc. In the neutral unloaded position, there were small but not significant differences in disc pressure for the four states of the treated disc (P>0.05). Concerning the disc pressure during the course of loading, both the Dynesys and internal fixator did significantly reduce the pressure change from neutral to extension in comparison to the intact state (-0.05, -0.04 and +0.24 MPa, respectively) (P<0.05). Compared to the intact state, there was no significant pressure change from neutral to flexion (0.14, 0.15 and 0.18 MPa, respectively) (P>0.05). The devices apparently eliminated the pressure change from neutral to lateral bending (Dynesys 0.01 MPa, Fixator 0.01 MPa and intact 0.24 MPa), but due to large variations in the intact and defect states the differences were not significant (P>0.05). In axial rotation, the pressure change for the internal fixator was reduced compared to the intact state; however, the change was only significant in left axial rotation (P<0.05). The Dynesys showed no significant differences (P>0.05) in axial rotation. No changes in IDP were seen in the adjacent discs for either the Dynesys or the internal fixator. Our results showed that the IDPs for both devices were similar, but altered compared to the intact disc.