[Intramedullary femoral osteosynthesis with the "rendezvous procedure". A new philosophy of retrograde intramedullary nailing]

OBJECTIVE

The disadvantages of retrograde intramedullary nailing are the necessity of explanting the nail through the knee and the occasional difficulties with proximal interlocking. The new retrograde intramedullary nail includes the following innovations: The nail is inserted into the medullary canal beyond the great trochanter, so that a proximal aiming device can be used. The nail is removed through the proximal opening. The distal opening can be closed with the osteochondral cylinder that has been taken out before.

MATERIAL AND METHODS

In all, 27 patients have been included in this prospective study. The intraoperative data of the implantation has been recorded. A clinical/radiologic examination took place on the day of discharge and 6-12-26 weeks after the operation in order to record the postoperative complications.

RESULTS

The mean time of follow-up examinations at the moment is 8.9 months (2-14). No infections have occurred. The healing of the fractures was without any problems. No patient complained of any major problems concerning the knee and the hip.

CONCLUSIONS

The new Sinart-Nail seems to be a nail that can be implanted easily and without complications. It combines the advantages of the retrograde intramedullary nailing with the advantages of the anterograde explantation. The two aiming devices facilitate a distal and proximate fixation without fluoroscopy. The replanting of the osteochondral cylinder reduces internal damage to the knee.

[Stabilizing effect and sintering tendency of 3 different cages and bone cement for fusion of cervical vertebrae segments]

Important requirement for spinal fusion devices for segment are that they provide sufficient stability and guarantee a low subsidence risk. An important requirement for spinal fusion devices for segments are that they provide sufficient stability and guarantee a low subsidence risk. Therefore, in the following in vitro study, the stabilizing effect and subsidence tendency of cervical fusion cages and bone cement were investigated during cyclic loading. The WING cages (Medinorm AG) and BAK cages (Spinetec) made of titanium, the carbon fiber reinforced PEEK cage from Acromed (DePuy Acromed), and bone cement (PMMA, Sulzer) were tested. Twenty-four human cervical spine specimens were first tested intact with a standardized flexibility test (+/- 2.5 Nm). Then the implants were inserted and the primary stability determined. For the simulation of the postoperative loading of the cervical spine a cyclic loading protocol with 700 loading cycles was performed. In this test pure moments +/- 2.0 Nm in 9 different loading directions in randomized order were applied together with a 50 N preload to simulate the weight of the head. The subsidence and "long term stability" was measured after 50, 100, 200, 300, 500, and 700 cycles. All implants had a stabilizing effect in all directions most obviously in lateral bending. Here the range of motion was between 20.9% (AcroMed Cage), and 62% (BAK Cage) with respect to the intact specimen (100%). In laterial bending, flexion, and axial rotation the AcroMed cage stabilized the most followed by the bone cement, WING and BAK Cage. In extension the specimens treated with bone cement were the most stable. After 700 loading cycles the specimens with the BAK cage lost 1.6 mm in height, with the WING Cage 0.8 mm, with the Acromed 0.7 mm, and with the bone cement 0.5 mm. Two Acromed Cages dislocated during the long term testing. Cages have the potential to stabilize as effectively as bone cement. A smaller contact area, however, causes a higher subsidence risk compared to bone cement but increases the fusion area, thus increasing the chance of obtaining bony fusion.

[Effect of artificial disk nucleus implant on mobility and intervertebral disk high of an L4/5 segment after nucleotomy]

This study investigated whether after a nucleotomy and implantation of a prosthetic disk nucleus (PDN) the original height and mobility of an L4/L5 disk can be restored. Compared to the intact state (100%), nucleotomy increased the median values of the normalized range of motion (ROM) in flexion/extension to 118%, lateral bending to 112%, and axial rotation to 121%. PDN implantation reduced ROM to 102%, 88%, and 90%. These differences were even more distinct when comparing the neutral zone (NZ) with 210%, 173%, and 107% after nucleotomy and 146%, 149%, and 44% after PDN implantation. With an axial preload of 200 N, disk height after nucleotomy was reduced by about 1.3 mm and could be restored with PDN implantation. PDN implantation can restore disk height and ROM after nucleotomy to normal values and reduce the strong NZ increase. Further biomechanical characterization of this therapy with PDN is necessary.

[Dislocation tendency, stabilizing effect and sintering tendency of different lumbar vertebrae cages in an in vitro experiment]

For biomechanical purposes, interbody fusion cages should not dislocate, should provide high stability, and should have a low subsidence risk. Zientek (Marquardt Medzintechnik), Stryker (Stryker Implants), and Ray lumbar interbody fusion cages (Surgical Dynamics) were tested in this study. They were implanted by pairs from a posterior approach without further stabilization. In a first step, each cage design was implanted into four human L3-4 segments and extracted posteriorly under an axial preload of 200 N. In a second step, standard flexibility tests were carried out with 24 human L2-3 and L4-5 specimens in an intact condition, directly after cage implantation, and after cyclic axial compression loading (200-1000 N, 40,000 cycles, 5 Hz). In a third step, a destructive axial compression test was carried out. Maximum pullout force was highest with Ray cages (median 945 N), followed by Zientek (605 N) and Stryker cages (130 N). With all three cage designs, primary stability was higher in lateral bending and flexion than in extension and axial rotation. Implantation of Ray cages caused a decreased range of motion in all three loading directions ranging between 49% and 99%. Zientek cages only stabilized in lateral bending, flexion, and extension (45-78%) and Stryker cages in none of the three loading directions. Cyclic loading caused an increased range of motion in all cases up to 190%. Axial compression force at failure was 8413 N with Ray cages, 8359 N with Stryker cages, and 5486 N with Zientek cages. The cage design seems to influence the dislocation tendency. In this regard, threaded cages or cages with anchorage systems seem to provide more security. The stabilizing effect seems to be mainly influenced by factors such as the degree of distraction or destruction of the facet joints rather than by the cage design.

[MACS-TL polyaxial screw XL. A new concept for increasing stability of ventral spondylodesis in the presence of dorsal injuries]

The influence of additional dorsal structure damage on anterior stabilization of thoracolumbar fracture is still unknown. Screw cement enhancement is a possibility to reinforce the stability of anterior instrumentation. A new anchorage system has been developed for fixation of anterior stabilization devices, adapted through geometric optimization and the possibility of optional additional cementation after screw insertion in cases of poor bone quality. Is this enhancement strong enough to support a single anterior procedure such as the thoracoscopic technique and still compensate for dorsal instability? A biomechanical in vitro study simulating an anterior corpectomy, strut grafting, and overbridging stabilization with a dorsal laminectomy as dorsal structure damage was performed, and the primary stability parameters were evaluated with and without screw cement enhancement. The additional cementation enhanced the primary stability of the anterior instrumentation and compensated for dorsal instability.

[Neon--a new angle-stable implant system for dorsal occipitocervical instrumentation. Biomechanical comparison with established systems]

Posterior instrumentation of the occipitocervical spine is well-established for different indications. The aim of this study was to evaluate whether posterior internal fixation of the occipitocervical spine with the new implant system improves primary biomechanical stability. Primary stability was significantly increased in all load cases with the new modular implant system compared to the other implant systems. Pedicle screw instrumentation tended to be stabler compared to lateral mass screws; nevertheless, significant differences could be observed only for lateral bending. As the experimental design precluded any cyclic testing, the data represent only the primary stability of the implants. In summary, this study showed that posterior instrumentation of the cervical spine using the new neon occipito-cervical system improves primary biomechanical stability compared to the CerviFix system and the Olerud cervical rod spinal system.

[MAC-TL twin screw. A new thoracoscopic implantable stabilization system for treatment of vertebral fractures--implant design, implantation technique and in vitro testing]

Due to the lack of an appropriate instrumentation system for minimally invasive procedures to treat spinal fracture, a new thoracoscopically implantable stabilization system was developed. This report describes the new implant design and implantation technique. In a biomechanical in vitro study, an anterior corpectomy model representing the worst case of burst fracture instability was simulated, and the primary stability parameters of the new system were evaluated in comparison to a dorsal stabilization system. With the interbody graft and fixation, the new system demonstrated higher stabilizing effects in flexion/extension and lateral bending and restored axial stability beyond the intact spine and the dorsal stabilization system. Considering all the advantages of the endoscopic procedure and this biomechanical characterization, the clinical trial is warranted; its usefulness has been demonstrated in more than 150 cases in a multicenter study to date.

Mechanically simulated muscle forces strongly stabilize intact and injured upper cervical spine specimens

Although muscles are assumed to be capable of stabilizing the spinal column in vivo, they have only rarely been simulated in vitro. Their effect might be of particular importance in unstable segments. The present study therefore tests the hypothesis that mechanically simulated muscle forces stabilize intact and injured cervical spine specimens. In the first step, six human occipito-cervical spine specimens were loaded intact in a spine tester with pure moments in lateral bending (+/- 1.5 N m), flexion-extension (+/- 1.5 N m) and axial rotation (+/- 0.5 N m). In the second step, identical flexibility tests were carried out during constant traction of three mechanically simulated muscle pairs: splenius capitits (5 N), semispinalis capitis (5 N) and longus colli (15 N). Both steps were repeated after unilateral and bilateral transection of the alar ligaments. The muscle forces strongly stabilized C0-C2 in all loading and injury states. This was most obvious in axial rotation, where a reduction of range of motion (ROM) and neutral zone to <50% (without muscles=100%) was observed. With increasing injury the normalized ROM (intact condition=100%) increased with and without muscles approximately to the same extend. With bilateral injury this increase was 125-132% in lateral bending, 112%-119% in flexion-extension and 103-116% in axial rotation. Mechanically simulated cervical spine muscles strongly stabilized intact and injured cervical spine specimens. Nevertheless, it could be shown that in vitro flexibility tests without muscle force simulation do not necessarily lead to an overestimation of spinal instability if the results are normalized to the intact state.

The effect of micromovement on callus formation

Micromovement at fracture sites is known to promote callus formation and bridging of the bony fragments. The present study was conducted to identify the suitable amount of micromovement, and to analyze the location and timing of callus proliferation. A standardized transverse osteotomy, in the right metatarsus of 32 sheep, was used as a fracture model. The osteotomy was externally fixed with a special ring fixator, which allowed axial micromovements of defined sizes. The animals were divided into four groups, with gaps of 2 mm and 6 mm, and micromovements of 0.3 mm and 0.7 mm, respectively. The labeling of new bone formation was performed by the intravenous injection of calcein green in the fourth week and tetracycline in the eighth week. Nine weeks postoperatively the sheep were killed. The explanted metatarsals were radiographed for the measurement of the periosteal callus area and were nondestructively loaded in a three-point bending test to determine their flexural rigidity. Histological analysis of undecalcified bone was performed in bone slices in the sagittal plane. Fluorescent green (callus formed in the fourth week) and yellow areas (callus formed in the eighth week) and the area of connective tissue were determined, using fluorescence microscopy. Bone formation was larger in the eighth week than that in the fourth week in all groups. In the fourth week, large micromovements in the small gap resulted in increased bone formation, whereas, for large gaps, the large micromovements diminished new bone formation. With large micromovement, the amount of newly formed bone within the gap decreased with increasing gap size, suggesting a delay of bone healing. Stimulation of new bone formation by micromovement was mainly effective in the early healing phase (4 weeks postoperatively). Large gaps showed the least new bone formation at the fracture site and the lowest flexural rigidity. From the histological analysis, it was found that the flexural rigidity correlated with the new bone area in the periosteal region.

Histomorphological, histomorphometrical and biomechanical analysis of ceramic bone substitutes in a weight-bearing animal model

It was the purpose of this investigation to prove the biomechanical properties, the osteoconductive capacity and the degradation rate of alpha tricalcium phosphate (alpha TCP), a neutralized glass ceramics (GB9N) and a composite material (GB9N+copolymers). In a weight-bearing animal model six substitutes each were implanted in the medial tibial head of the right lower leg of adult Merino-sheep in a standardized surgical technique. After nine months the implants were harvested and prepared for histomorphological and histomorphometrical investigations (undecalcified Masson Goldner staining). For additional biomechanical testing of the specimens, non-operated bone blocks from the contralateral tibia as well as native implants served as controls. No significant differences for the maximum fracture load as well as for the yield strength were detected between harvested specimens and bone blocks from the contralateral tibia. However there were marked differences to ceramics that were not implanted. All substitutes showed osteoconduction, leading to a continuous ingrowth of new formed bone. However in the composite material soft tissue could be identified within the scaffold and there were signs of ongoing bone remodeling, nine months after implantation. The bone per tissue volume of alpha-TCP in conjunction to new bone (=percentage of trabecular bone volume plus percentage of residual substitute) was higher than for GB9N and the composite material. Nine months after implantation the percentage of residual alpha-TCP was 48%, it was 32% for GB9N and 28% for the composite. The intention of further studies should be to accelerate the degradation rates of substitutes and to improve biomechanical properties of implants by either modifying the chemical composition or combining materials with agents as, e.g. growth factors.

Influence of a follower load on intradiscal pressure and intersegmental rotation of the lumbar spine

STUDY DESIGN

Intradiscal pressure and intersegmental rotation of human lumbar spines were measured in vitro.

OBJECTIVES

To determine the effect of a follower load on mechanical behavior at all levels of the lumbar spine.

SUMMARY OF BACKGROUND DATA

Different loads have been proposed for studying the mechanical behavior of the lumbar spine. The influence of a follower load on intradiscal pressure at the different levels is unknown.

METHODS

Ten human cadaveric lumbar spines were loaded in the three main anatomic planes with pure moments of 3.75, 7.5, and 7.5 Nm plus a follower load of 280 N. Intradiscal pressure and intersegmental rotation were measured at all levels.

RESULTS

An additional follower load increased the intradiscal pressure, slightly reduced the intersegmental rotation for axial rotation, and hardly affected intersegmental rotation for lateral bending and flexion-extension.

CONCLUSIONS

A superimposed follower load renders spinal loading with pure moments more physiologic.

Resistance of the lumbar spine against axial compression forces after implantation of three different posterior lumbar interbody cages

BACKGROUND

The aim of using interbody fusion cages is to distract the degeneratively decreased disc height to decompress the neural structures in the intervertebral foramina and allow bony fusion. Prerequisite for a successful fusion therapy is a high resistance against subsidence and breakage.

METHOD

Three types of implants, a cylindrical threaded titanium cage (Ray) (1c), a bullet shaped PEEK cage (Stryker) (1a) and a rectangular titanium cage with an endplate anchorage device (Marquardt) (1b) were implanted in eight monosegmental lumbar spine specimens (L 2/3 and L 4/5). Each specimen underwent a cyclic loading test with 40000 cycles at a rate of 5 Hz. A cyclic axial compression force ranging from 200 Newton [N] to 1000 N was applied and the axial translation recorded simultaneously to determine the subsidence tendency. After this procedure the specimens were tested with a progressive axial force until breakage.

FINDINGS

There were only small differences in the subsidence tendency for the three cage designs. The height reduction due to cyclic loading ranged between 0.9 mm (Marquardt), 1.2 mm (Stryker) and 1.4 mm (Ray). The median break force ranged from 5486 N (Marquardt), 8359 N (Stryker) to 8413 N (Ray). No correlation between bone mineral density and failure load could be detected.

INTERPRETATION

Endplate preparation and cage design of the tested implants do not seem to influence the resistance of the segment against cyclic axial compression. The compression with a continuously increasing load revealed that an implant-bone failure is not to be expected in physiological limits for all three cage types.

Intradiscal pressure together with anthropometric data--a data set for the validation of models

OBJECTIVE

To provide a database of intradiscal pressure measurements together with anthropometric data as basis for the validation of models that predict spinal loads.

DESIGN

Intradiscal pressure was measured in a non-degenerated L4-5 disc of a volunteer. The anthropometric characteristics of this subject were extensively determined.

BACKGROUND

Since it is usually impossible to quantify the load in the spine directly, it is predicted by various biomechanical models. However, they often cannot be validated because of the few in vivo data and missing anthropometric characteristics pertaining to them.

METHODS

A pressure transducer (diameter 1.5 mm) was implanted in the nucleus pulposus of a non-degenerated L4-5 disc of a volunteer. Pressure was determined during exercises while standing, lifting activities, sitting unsupported on a stool or an ergonomic sitting ball, sitting in different postures and others. The anthropometric characteristics were determined using different tools.

RESULTS

Pressure values: relaxed standing 0.5 MPa; standing flexed forward 1.1 MPa; standing extended backward 0.6 MPa; sitting unsupported 0.46 MPa; maximum values during lateral bending 0.6 MPa, during axial rotation 0.7 MPa, lifting a 20 kg weight with a round flexed back 2.3 MPa, with flexed knees 1.7 MPa, close to the body 1.1 MPa; sitting unsupported relaxed 0.45 MPa, actively straightening the back 0.55 MPa, with flexion 0.9 MPa; non-chalant sitting 0.3 MPa and others. Anthropometric characteristics with emphasis on data for the trunk are provided in tables.Conclusions. Intradiscal pressure depends on the kind of preceding activity, posture, external loads and muscle activity.

RELEVANCE

The data set can be used to verify a biomechanical model adjusted to the individual characteristics by a comparison of measured and predicted intradiscal pressures.

Influence of femoral anteversion on proximal femoral loading: measurement and simulation in four patients

OBJECTIVE

The aim of this study was to determine the loading of the proximal femur during daily activities and to quantify the influence of femoral anteversion.

DESIGN

This study combined experimental and analytical approaches to determine the in vivo loading at the hip joint. A numerical musculo-skeletal model was validated against measured in vivo hip contact forces and then used to analyse the influence of anteversion on the loading conditions in the femur.

BACKGROUND

Musculo-skeletal loading of long bones is essential for joint replacement and fracture healing. Although joint contact forces have previously been measured in selected patients, the interaction between femoral anteversion and the associated musculo-skeletal loading environment remains unknown.

METHODS

The gait of four patients with force measuring hip prostheses was analysed during walking and stair-climbing. Musculo-skeletal loading was determined using individual numerical models by minimising the sum of the muscle forces.

RESULTS

Experimentally and numerically determined hip contact forces agreed both qualitatively and quantitatively. Muscle activity resulted in compression of the femur and small shear forces in the meta- and epi-physeal regions. Increasing the anteversion to an angle of 30 degrees increased hip contact forces and bending moments up to 28%.

CONCLUSIONS

This study has shown that femoral anteversion has a strong influence on the musculo-skeletal loading environment in the proximal femur.

RELEVANCE

Detailed musculo-skeletal modelling may allow pre-surgical, patient specific optimisation of loading on implant, bone and soft tissues.

Solvent dehydrated bone transplants to bridge segmental bone defects: histomorphological and biomechanical investigations in an animal model

Cancellous bone is routinely used in human surgery to fill skeletal defects. The availabilty of autogenous and allogenous grafts is limited, however. The aim of this in vivo study was therefore to determine the in-growth behaviour and biomechanical properties of solvent dehydrated human bone as an alternative to the use of autografts. In a weight-bearing experimental model, solvent dehydrated bone transplants were implanted subchondrally in the medial proximal tibia of merino sheep. After 9 months, explants as well as controls from the contralateral leg were harvested and prepared for histomorphological, histomorphometrical and biomechanical examination. A smaller, but statistically insignificant difference was found for the yield strength after 9 months for harvested specimens in comparison with untreated controls. Regarding the histomorphological results, we found a homogenous ingrowth of new bone trabeculae throughout the transplants. The degradation of the solvent dehydrated bone was not complete within the study period as shown by persistent bone remodelling. The bone per tissue volume of remaining solvent dehydrated graft particles together with newly formed bone was significantly higher than for controls. Our observation period was not long enough to document complete remodelling, but good osteointegration and reasonable biomechanical properties in this weight-bearing large animal model support the application of solvent dehydrated bone in cancellous defects of clinical relevance.

Radiographic results of callus distraction aided by pulsed low-intensity ultrasound

OBJECTIVES

To determine whether pulsed low-intensity ultrasound (frequency of 1.5 megahertz, pulsed by one kilohertz, signal burst width of 200 microseconds, intensity of thirty milliwatts per square centimeter, and daily treatment time of twenty minutes per day) stimulates regenerate maturation after callus distraction.

DESIGN

Prospective, controlled animal trial.

METHODS

Operatively, we created a fifteen-millimeter defect in the right metatarsus of eighteen female mature merino sheep. A segmental transport was begun on Day 5 using a high-stiffness experimental ring fixator. The distraction rate was one millimeter per day divided into two increments of 0.5 millimeters each. On Day 21 after the operation, distraction was finished and the maturation period started and lasted until Day 84 after operation. During this period, Group 1 was treated with a daily twenty-minute low-intensity ultrasound stimulation (frequency of 1.5 megahertz, pulsed by one kilohertz, signal burst width of 200 microseconds, intensity of thirty milliwatts per square centimeter). Group 2 had no stimulation. Animals bore full weight. Plain radiographs in the anteroposterior view were taken every two weeks during the maturation period. After the animals were killed on Day 84, anteroposterior and lateral high resolution radiographs and computed tomography (CT) scans of the regenerate were performed. For each plain and high resolution radiograph, two different relationships (callus relation, the ratio of the amount of periosteal callus to the size of the space between the proximal fragment and transported segment; and interzone relation, the ratio of the fibrous callus interzone to the size of the new formed callus) were calculated. Using CT scan, callus area, bone density, and bone mineral content were evaluated.

RESULTS

The results of interzone relation (both views) and callus relation (lateral view) in high-resolution radiographs and bone mineral content in CT indicate a significantly accelerated maturation of the regenerate in the ultrasound stimulated group even when a Bonferroni-Holm adjustment was used for multiple testing.

CONCLUSION

Pulsed low-intensity ultrasound appears to stimulate the healing processes in the regenerate in this animal model and may have applicability in clinical practice.

Suitability of external fixators for use in the tropics

External fixation systems proved to decrease the osteomyelitis rate in patients in the tropics compared with internal stabilization. This study was designed to show how external fixators being used for treatment of patients in industrial countries compare with cheap alternatives regarding their suitability for the application in tropical countries. Eleven external fixation systems were compared for stability, cost, weight, variability, handling, and capability of being produced locally. Stiffness, slipping moment, and irreversible deformation were determined in material testing machines. The technically best fixators are expensive and cannot be manufactured locally. The inexpensive constructs lack variability and stability. When cost is not a problem, the Synthes model is recommended. With some restrictions in mechanical stability and variability, the Pfeifer Fixator II and a wooden model offer inexpensive and locally producible alternatives. These results may help to select an external fixation device to meet local needs and possibilities in tropical countries.

Musculo-skeletal loading conditions at the hip during walking and stair climbing

Musculo-skeletal loading plays an important role in the primary stability of joint replacements and in the biological processes involved in fracture healing. However, current knowledge of musculo-skeletal loading is still limited. In the past, a number of musculo-skeletal models have been developed to estimate loading conditions at the hip. So far, a cycle-to-cycle validation of predicted musculo-skeletal loading by in vivo measurements has not been possible. The aim of this study was to determine the musculo-skeletal loading conditions during walking and climbing stairs for a number of patients and compare these findings to in vivo data. Following total hip arthroplasty, four patients underwent gait analysis during walking and stair climbing. An instrumented femoral prosthesis enabled simultaneous measurement of in vivo hip contact forces. On the basis of CT and X-ray data, individual musculo-skeletal models of the lower extremity were developed for each patient. Muscle and joint contact forces were calculated using an optimization algorithm. The calculated peak hip contact forces both over- and under-estimated the measured forces. They differed by a mean of 12% during walking and 14% during stair climbing. For the first time, a cycle-to-cycle validation of predicted musculo-skeletal loading was possible for walking and climbing stairs in several patients. In all cases, the comparison of in vivo measured and calculated hip contact forces showed good agreement.Thus, the authors consider the presented approach as a useful means to determine valid conditions for the analysis of prosthesis loading, bone modeling or remodeling processes around implants and fracture stability following internal fixation.

De novo mutations in the sodium-channel gene SCN1A cause severe myoclonic epilepsy of infancy

Severe myoclonic epilepsy of infancy (SMEI) is a rare disorder that occurs in isolated patients. The disease is characterized by generalized tonic, clonic, and tonic-clonic seizures that are initially induced by fever and begin during the first year of life. Later, patients also manifest other seizure types, including absence, myoclonic, and simple and complex partial seizures. Psychomotor development stagnates around the second year of life. Missense mutations in the gene that codes for a neuronal voltage-gated sodium-channel alpha-subunit (SCN1A) were identified in families with generalized epilepsy with febrile seizures plus (GEFS+). GEFS+ is a mild type of epilepsy associated with febrile and afebrile seizures. Because both GEFS+ and SMEI involve fever-associated seizures, we screened seven unrelated patients with SMEI for mutations in SCN1A. We identified a mutation in each patient: four had frameshift mutations, one had a nonsense mutation, one had a splice-donor mutation, and one had a missense mutation. All mutations are de novo mutations and were not observed in 184 control chromosomes.

Mechanical boundary conditions of fracture healing: borderline indications in the treatment of unreamed tibial nailing

Unreamed nailing favors biology at the expense of the achievable mechanical stability. It is therefore of interest to define the limits of the clinical indications for this method. The extended usage of unreamed tibial nailing resulted in reports of an increased rate of complications, especially for the distal portion of the tibia. The goals of this work were to gain a thorough understanding of the load-sharing mechanism between unreamed nail and bone in a fractured tibia, to identify the mechanical reasons for the unfavorable clinical results, and to identify borderline indications due to biomechanical factors. In a three-dimensional finite element model of a human tibia, horizontal defects were stabilized by means of unreamed nailing for five different fracture locations, including proximal and distal borderline indications for this treatment method. The loading of the bone, the loading of the implant and the inter-fragmentary strains were computed. The findings of this study show that with all muscle and joint contact forces included, nailing leads to considerable unloading of the interlocked bone segments. Unreamed nailing of the distal defect results in an extremely low axial and high shear strain between the fragments. The results suggest that mechanical conditions are advantageous to unreamed nailing of proximal and mid-diaphyseal defects. Apart from biological reasons, clinical problems reported for distal fractures may be due to the less favorable mechanical conditions in unreamed nailing. From a biomechanical perspective, the treatment of distal tibial shaft fractures by means of unreamed nailing without additional fragment contact or without stabilizing the fibula should be carefully reconsidered.

[Intra-articular and plantar pressure distribution of the ankle joint complex in relation to foot position]

Ankle injuries are often followed by degenerative changes in the hindfoot joints. Knowledge about the pressure distribution of the intact ankle joint may help to understand the mechanisms leading to cartilage damage. Therefore, we determined the intraarticular and plantar pressure distribution of the ankle joint complex and the Chopart joints with varying foot positions. 12 human lower leg specimens were axially loaded in a foot-loading simulator with full body weight (600 N). A capacitive pressure distribution platform was used to determine plantar pressure patterns. The intraarticular loading situation was measured with Fuji Prescale film. 3 different foot positions (neutral, 10 degrees dorsiflexion, 10 degrees plantarflexion) were investigated. Dorsiflexion led to an increase of the intraarticular contact area, force and mean pressure in the hindfoot. Plantarflexion instead increased loading in the Chopart joints. In the plantar pressure distribution force and peak pressure under the hindfoot increased with dorsiflexion. With plantarflexion area, force and peak pressure under mid- and forefoot increased. With our study we could demonstrate that the loading situation of the ankle joint complex is significantly influenced by the foot position. These findings may help to understand the development and localisation of arthritic changes due to posttraumatic changes of the joint loading characteristics.

Prediction of strength of cortical bone in vitro by microcomputed tomography

OBJECTIVE

The aim of this study was to evaluate the predictive value of bone mineral density and intracortical porosity measured by microcomputed tomography for the strength of cortical bone biopsies.

DESIGN

Experimental study comparing the predictive value of bone mineral density and of intracortical porosity determined in vitro by microcomputed tomography for the mechanical properties of cortical bone cylinders.

BACKGROUND

The assessment of cortical bone strength might be relevant for the prediction of fracture risk or the choice of suitable therapy strategies in orthopaedic surgery. The predictive value of cortical density for the mechanical properties is discussed controversially. The relevance of intracortical porosity measured by histomorphometry has been established, but the predictive value of porosity determined by microcomputed tomography remains to be explored.

METHODS

Femoral cortical bone specimens from the mid diaphysis of 24 patients were harvested during total hip replacement procedure at the location, where a diaphyseal hole (diameter 4.5 mm) was drilled in order to reduce the intramedullary pressure. In vitro intracortical porosity and bone mineral density measurements by microcomputed tomography were compared with strength and elastic modulus assessed by a compression test transverse to the Haversian systems of the same specimens.

RESULTS

Significant negative correlations were found between porosity measured by microcomputed tomography scans and yield stress, stiffness and elastic modulus (P<0.001), however, the positive correlations between bone mineral density and mechanical parameters were stronger (P<0.0001). The mechanical parameter best predicted by mineral density as well as by porosity was yield stress (r=0.72,P<0.0001;r=-0.64,P<0.001).

CONCLUSIONS

Bone mineral density determined by microcomputed tomography imaging in vitro may be a potent method to predict mechanical properties of cortical bone non-destructively. The application in vivo remains to be explored.

Cell alignment is induced by cyclic changes in cell length: studies of cells grown in cyclically stretched substrates

Many types of cells, when grown on the surface of a cyclically stretched substrate, align away from the stretch direction. Although cell alignment has been described as an avoidance response to stretch, the specific deformation signal that causes a cell population to become aligned has not been identified. Planar surface deformation is characterized by three strains: two normal strains describe the length changes of two initially perpendicular lines and one shear strain describes the change in the angle between the two lines. The present study was designed to determine which, if any, of the three strains was the signal for cell alignment. Human fibroblasts and osteoblasts were grown in deformable, rectangular, silicone culture dishes coated with ProNectin, a biosynthetic polymer containing the RGD ligand of fibronectin. 24 h after plating the cells, the dishes were cyclically stretched at 1 Hz to peak dish stretches of 0% (control), 4%, 8%, and 12%. After 24 h of stretching, the cells were fixed, stained, and their orientations measured. The cell orientation distribution was determined by calculating the percent of cells whose orientation was within each of eighteen 5 degrees angular intervals. We found that the alignment response was primarily driven by the substrate strain which tended to lengthen the cell (axial strain). We also found that for each cell type there was an axial strain limit above which few cells were found. The axial strain limit for fibroblasts, 4.2 +/- 0.4%, (mean +/- 95% confidence), was lower than for osteoblasts, 6.4 +/- 0.6%. We suggest that the fibroblasts are more responsive to stretch because of their more highly developed actin cytoskeleton.

Augmentation of a ruptured posterior cruciate ligament provides normal knee joint stability during ligament healing

OBJECTIVE

To identify an augmentation technique which would provide mechanical protection for the healing posterior cruciate ligament.

DESIGN

Six human knee specimens were tested in vitro for posterior knee joint stability after augmenting the cut posterior cruciate ligament by six different techniques using a resorbable double strand Polydioxanone augmentation device.

BACKGROUND

A fresh isolated rupture of the posterior cruciate ligament is often treated conservatively. Results have shown that it can heal, but ligament elongations occur frequently. Therefore a method is needed to provide posterior knee joint stability during ligament healing.

METHODS

The effect of different femoral augmentation insertions on posterior knee stability was tested by recording the antero-posterior (AP) position of the tibia and the augmentation force. Testing was performed during flexion--extension cycles and under posterior shear loads.

RESULTS

The insertion combination that proved to stabilize the joints best consisted of one augmentation strand leading along the antero-lateral posterior cruciate ligament fibres and inserting at the distal end of the Blumensaat line and one strand leading along the posteriormedial fibres and inserting in the middle of the Blumensaat line. AP translations similar to those occurring in healthy knee joints could be achieved.

CONCLUSIONS

It is possible to restore normal posterior knee joint stability by implanting a double strand augmentation device. This can help a posterior cruciate ligament to heal under non-elongated conditions.

Predictive value of Singh index and bone mineral density measured by quantitative computed tomography in determining the local cancellous bone quality of the proximal femur

OBJECTIVE

The purpose of this study was to assess the predictive value of the Singh index as well as quantitative computed tomography for the in vitro local mechanical competence of the cancellous bone of the proximal femur.

DESIGN

An experimental study examining the relation between mechanical properties and bone mineral density of the femoral neck determined in vitro and the clinical estimated Singh index on X-rays.

BACKGROUND

Evaluation of the predictive value of the Singh index, an inexpensive and simple technique for the mechanical properties of the cancellous bone of the proximal femur.

METHODS

The bone quality of the proximal femur of 34 patients undergoing total hip replacement was estimated by roentgenography using the Singh index. Bone mineral density was quantified by quantitative computed tomography using cylindrical cancellous bone biopsies harvested during the total hip replacement procedure by a new biopsy method. The mechanical properties of the bone specimens (Young's modulus, strength and maximum energy absorption E(max)) were measured by mechanical testing of the bone biopsies.

RESULTS

A strong correlation of the Singh index versus material properties of cancellous bone was noted (r=0.66 for Young's modulus, r=0.73 for strength and r=0.69 for E(max), P<0.0001). The correlations of bone mineral density measured by quantitative computed tomography versus Young's modulus, strength and energy absorption E(max) were significant. Strength was predicted best (r=0.82; P<0.0001), followed by E(max) (r=0.79; P<0.0001) and Young's Modulus (r=0.73; P<0.0001).

CONCLUSIONS

We conclude, that assessment of bone mineral density by quantitative computed tomography is a reliable and precise method for the estimation of cancellous bone material properties. The Singh index provides a rough estimate for the mechanical competence of the proximal femur. It is inexpensive, simply to assess and can in some cases replace the measurement of bone mineral density, notably in cases of marked decrease in bone density.

Prediction of cortical bone porosityIn Vitro by microcomputed tomography

The high importance of intracortical porosity for mechanical strength of cortical bone has been established. The contribution of other parameters of microstructure such as osteon dimensions for strength is in discussion. The aim of this study was to evaluate the predictive value of microcomputed tomography (µCT) for porosity and other microstructural parameters of cortical bone in cortical bone biopsies. Femoral cortical bone specimens from the middiaphysis of 24 patients were harvested during the procedure of total hip replacement at the location where normally one hole (Ø 4.5 mm) for the relief of the intramedullary pressure is placed.In vitro intracortical porosity and bone mineral density (BMD) measurements by µCT were compared with structural parameters assessed in histological sections of the same specimens. A strong correlation was found between intracortical porosity measured by µCT and histological porosity (r=0.95,P<0.0001). Porosity measured by µCT was also a strong predictor for other parameters describing dimensions of porous structures. BMD^-1 was associated with osteonal area (r=-0.76,P<0.0001). We consider the measurement of porosity by µCT as a very potent procedure for assessing intracortical porosity and parameters related to porous structures of cortical bone nondestructivelyin vitro.

Mechanical stimulation by external application of cyclic tensile strains does not effectively enhance bone healing

OBJECTIVE

To determine whether an externally induced interfragmentary movement enhances the healing process of a fracture under flexible fixation.

DESIGN

Randomized, prospective in vivo animal study with control group. Twenty-four skeletally mature Merino sheep were randomly assigned to six groups of four animals, which received cyclic interfragmentary movements of 0.2 and 0.8 millimeters and stimulation frequencies of 1, 5, and 10 Hertz, respectively. Twelve animals did not receive any externally applied stimulation and served as a control group.

SETTING

Unrestricted stall activity with weight bearing reduced by tenotomy of the Achilles tendon.

INTERVENTIONS

Osteotomy of the tibial diaphysis with three-millimeter gap width fixed with a six-pin, monolateral, double-bar external fixator. Interfragmentary movement of the osteotomy gap was externally induced by a motor-driven actuator unit. Five hundred cycles inducing nonuniform tensile strains within the gap were performed each day.

MAIN OUTCOME MEASUREMENTS

Nine weeks after surgery, the animals were killed, and bone mineral density and callus cross-sectional area were measured with quantitative computed tomography. Callus projectional area was assessed by radiographs, and mechanical stability was determined with a three-point bending test.

RESULTS

External stimulation with nonuniform cyclic tensile strains did slightly affect but not significantly enhance the fracture healing process. Varying the stimulation frequency had no influence on the healing process. The stimulation with 0.8 millimeter displacement magnitude resulted in a larger periosteal callus, but a decreased bone mineral density compared with the 0.2-millimeter displacement magnitude. The stimulation had no significant influence on the mechanical properties of the healing bone.

CONCLUSIONS

Induced cyclic tensile strains did not produce a relevant enhancement of bone healing under flexible fixation.

Effects of neck movements on stability and subsidence in cervical interbody fusion: an in vitro study

OBJECT

The aim of this in vitro study was to determine the influence of simulated postoperative neck movements on the stabilizing effect and subsidence of four different anterior cervical interbody fusion devices. Emphasis was placed on the relation between subsidence and spinal stability.

METHODS

The flexibility of 24 human cervical spine specimens was tested before and directly after being stabilized with a WING, BAK/C, AcroMed I/F cage, or with bone cement in standard flexibility tests under 50 N axial preload. Thereafter, 700 pure moment loading cycles (+/- 2 Nm) were applied in randomized directions to simulate physiological neck movements. Additional flexibility tests in combination with measurements of the subsidence depth were conducted after 50, 100, 200, 300, 500, and 700 loading cycles. In all four groups, simulated postoperative neck movements caused an increase of the range of motion (ROM) ranging from 0.4 to 3.1 degrees and of the neutral zone from 0.1 to 4.2 degrees. This increase in flexibility was most distinct in extension followed by flexion, lateral bending, and axial rotation. After cyclic loading, ROM tended to be lower in the group fitted with AcroMed cages (3.3 degrees in right lateral bending, 3.5 degrees in left axial rotation, 7.8 degrees in flexion, 8.3 degrees in extension) and in the group in which bone cement was applied (5.4 degrees, 2.5 degrees, 7.4 degrees, and 8.8 degrees, respectively) than in those fixed with the WING (6.3 degrees, 5.4 degrees, 9.7 degrees, and 6.9 degrees, respectively) and BAK cages (6.2 degrees, 4.5 degrees, 10.2 degrees, and 11.6 degrees, respectively).

CONCLUSIONS

Simulated repeated neck movements not only caused an increase of the flexibility but also subsidence of the implants into the adjacent vertebrae. The relation between flexibility increase and subsidence seemed to depend on the implant design: subsiding BAK/C cages partially supported stability whereas subsiding WING cages and AcroMed cages did not.

Prediction of cortical bone porosity in vitro by microcomputed tomography

The high importance of intracortical porosity for mechanical strength of cortical bone has been established. The contribution of other parameters of microstructure such as osteon dimensions for strength is in discussion. The aim of this study was to evaluate the predictive value of microcomputed tomography (mCT) for porosity and other microstructural parameters of cortical bone in cortical bone biopsies. Femoral cortical bone specimens from the middiaphysis of 24 patients were harvested during the procedure of total hip replacement at the location where normally one hole (Ø 4.5 mm) for the relief of the intramedullary pressure is placed. In vitro intracortical porosity and bone mineral density (BMD) measurements by mCT were compared with structural parameters assessed in histological sections of the same specimens. A strong correlation was found between intracortical porosity measured by mCT and histological porosity (r = 0.95, P <0.0001). Porosity measured by mCT was also a strong predictor for other parameters describing dimensions of porous structures. BMD?1 was associated with osteonal area (r = -0.76, P <0.0001). We consider the measurement of porosity by mCT as a very potent procedure for assessing intracortical porosity and parameters related to porous structures of cortical bone nondestructively in vitro.

In vitro effects of dynamic strain on the proliferative and metabolic activity of human osteoblasts

AIM OF THE STUDY

It has been well shown by human and animal studies that mechanical load is an important regulator of skeletal mass and architecture. However, cellular reactions which adapt bone tissue to the mechanical environment are not definitively determined. For this purpose we studied the cell activity of human bone derived cell cultures after mechanical stimulation by cyclic, uniaxial strain at a magnitude occurring in normal loaded bone tissue.

MATERIALS AND METHODS

Human osteoblasts were isolated from cancellous bone biopsies of 5 different donors. Cell seeding was made in DMEM in a density of 10.000 cells/cm(2) on deformable culture dishes for three days prior to initiating cell stretching at 1000 microstrain, 1Hz for 1800 cycles for two subsequent days with an especially developed cell stretching device. 48h after the second stimulation cells were harvested and cell number was determined with a Coulter Counter. Cell bound alkaline phosphatase activity was analyzed in cell lysates by a colorimetric assay, osteocalcin and CICP (procollagen I propeptide) production were analyzed in cell supernatants with ELISAs. Three parallel cultures were tested.

STATISTICS

Wilcoxon.

RESULTS

In all experiments mechanical stimulation resulted in a significant increase in cell number (10-48%) and CICP release (7-49%). Simultaneously a significant decrease in alkaline phosphatase activity (9-25%) and osteocalcin release (5-32%) could be observed.

CONCLUSIONS

The results demonstrate that cyclic strain at physiologic magnitude leads to an increase of early osteoblast activities related to matrix production while those activities which are characteristic for the differentiated osteoblast and relevant for matrix mineralization are decreased. These new findings confirm in vivo observations about the importance of dynamic strain for bone formation during fracture healing and bone remodeling and could contribute to the optimization of fracture healing.

Initial stability of fully and partially cemented femoral stems

OBJECTIVE

To test the initial stability of a newly designed partially cemented femoral stem in comparison with a fully cemented conventional stem.

DESIGN

An in vitro study to determine the interface motion between femoral stem and bone as a response to loading.

BACKGROUND

The aim of the new prosthesis design is a proximal load transfer by a defined partial cement fixation in the proximal femur region and a slim prosthesis stem in the distal region. Before a clinical study can be started, the new stem has to show an initial stability comparable to that of fully cemented prostheses.

METHOD

Six paired fresh cadaveric femora were used for the testing of the new partially cemented stem (Option 3000, Mathys Orthopaedics, Bettlach, Switzerland) and a fully cemented stem (Weber Shaft, AlloPro, Baar, Swizerland). Under cyclic loading up to 1600 N hip joint forces, the interface motion between implants and bone was measured at six locations.

RESULTS

Both stems showed uncritical interface motions below 43 microm. However, the Option 3000 stem exhibited significantly smaller motions in the proximal region and slightly larger movements in the distal regions than the Weber prosthesis.

CONCLUSIONS

The new type of partially cemented stem provided a comparable initial stability to the fully cemented Weber prosthesis. Relevance The high initial stability of the Option 3000 stem justified the clinical use of the new implant. More than 100 implantations in the last three years, with very good preliminary clinical results, support the preclinical findings.

Subsidence resulting from simulated postoperative neck movements: an in vitro investigation with a new cervical fusion cage

STUDY DESIGN

A biomechanical in vitro subsidence test of different cervical interbody fusion devices was performed using a new testing protocol that simulates physiologic conditions.

OBJECTIVES

To investigate the effect of simulated postoperative neck movements on the subsidence of the new WING cervical interbody fusion cage in comparison with two other cages and bone cement.

SUMMARY OF BACKGROUND DATA

Cervical interbody fusion cages sometimes cause complications because of subsidence into the adjacent vertebrae with collapse of the intervertebral space. Complications such as cage dislocation or nonunion with instability also have been reported. To prevent such complications, the new WING cervical interbody fusion cage (Medinorm AG, Quierschied, Germany) has been developed. Its area of contact with the adjacent vertebrae is supposed to be large enough to resist excessive subsidence and small enough to prevent stress protection of the tissue growing in the cage.

METHODS

In this study, 24 human cervical spine specimens were tested after stabilization with either a WING, BAK/C, AcroMed I/F cage or bone cement. Then, in a new testing protocol, 700 pure-moment loading cycles (+/-2 Nm) were applied in randomized directions (lateral bending, flexion-extension, and axial rotation alone or in combination with each other) to simulate the patient's neck movements during the first few postoperative days. Measurements of the subsidence depth (total height loss) in combination with flexibility tests (+/-2.5 Nm) were performed before cyclic loading and after 50, 100, 200, 300, 500, and 700 loading cycles.

RESULTS

Cyclic loading caused subsidence in all four device groups, most distinct with BAK/C-cages (1.63 mm after 700 loading cycles) followed by the new WING (0.90 mm) and the AcroMed (0.82 mm) cages. No statistically significant difference could be found among the three cage designs. However, all three cage types showed a significantly higher subsidence depth than bone cement (0.48 mm;P = 0.023 between each of the three cage-types and bone cement). A moderate correlation between bone mineral density and subsidence depth could be found only in the BAK/C group (r2 = 0.495). A large subsidence depth after 700 loading cycles was associated with a large flexibility increase in the WING (r2 = 0.786) and AcroMed groups (r2 = 0.21), but with a small flexibility increase in the BAK/C group (r2 = 0.58).

CONCLUSIONS

Postoperative neck movements caused subsidence in all cervical interbody implant types. The new WING cage and the AcroMed cage seemed to have a better resistance against subsidence than the BAK/C cage. However, all three cage types had a significantly higher subsidence tendency than bone cement.

Comparative stability of the "Internal Fixator" and the "Universal Spine System" and the effect of crosslinking transfixating systems. A biomechanical in vitro study

In this study, the three-dimensional stabilizing capabilities of the AO-Internal Fixator (IF) and the new Universal Spine System (USS) were investigated. Both devices were tested without and with the cross-link system (IF, IFC, USS, USSC). To determine biomechanical characteristics, a human thoracolumbar spine instability model with resection of the vertebral body Th12 was created. The vertebral body was replaced by a spacer and transpedicular posterior stabilization was performed from Th11 to L1. All devices reduced the range of motion (ROM) significantly compared to the values of the intact specimen. In flexion the IFC showed the highest reduction of ROM (85% of intact), followed by the USSC, USS and IF (79% of intact). In extension the ROM was restored again most by the IFC (52% of intact), followed by the USSC, IF and USS (44% of intact). In lateral bending stability was provided by the USSC (right 78% and left 81% of intact), followed in right lateral bending by the IF, IFC and USS and in left lateral bending by the USS, IF and IFC. In axial rotation the ROM was reduced primary by the IFC (right 51% and left 46% of intact), followed in right axial rotation by the USS, USSC and IF, in left axial rotation by the USSC, USS and IF. Additional stability by crosslinking has been provided in the IF and the USS in flexion and extension, in the USS in lateral bending and in the IF in axial rotation nonsignificantly. The neutral zone (NZ) was reduced by posterior instrumentation in flexion/extension and right/left lateral bending significantly. In axial rotation only the USSC decreased the NZ below intact levels. The study showed no statistical significant differences in the stabilizing capabilities of the USS compared to the IF. For both implants the cross-link system increased stability in the chosen instability model insignificantly only.

Primary stabilizing effect of interbody fusion devices for the cervical spine: an in vitro comparison between three different cage types and bone cement

Interbody fusion cages are small hollow implants that are inserted into the intervertebral space to restore physiological disc height and to allow bony fusion. They sometimes cause clinical complications due to instability, subsidence or dislocation. These are basic biomechanical parameters, which influence strongly the quality of a fusion device; however, only few data about these parameters are available. Therefore, the purpose of the present study was to investigate the primary stabilizing effect of four different cervical fusion devices in in vitro flexibility tests. Twenty-four human cervical spine segments were used in this study. After anterior discectomy, fusion was performed either with a WING cage (Medinorm AG, Germany), a BAK/C cage (Sulzer SpineTech, USA), an AcroMed cervical I/F cage (DePuy AcroMed International, UK) or bone cement (Sulzer, Switzerland). All specimens were tested in a spine tester in the intact condition and after implantation of one of the four devices. Alternating sequences of pure lateral bending, flexion-extension and axial rotation moments (+/- 2.5 Nm) were applied continuously and the motions in each segment were measured simultaneously. In general, all tested implants had a stabilizing effect. This was most obvious in lateral bending, where the range of motion was between 0.29 (AcroMed cage) and 0.62 (BAK/C cage) with respect to the intact specimen (= 1.00). In lateral bending, flexion and axial rotation, the AcroMed cervical I/F cages had the highest stabilizing effect, followed by bone cement, WING cages and BAK/C cages. In extension, specimens fused with bone cement were most stable. With respect to the primary stabilizing effect, cages, especially the AcroMed I/F cage but also the WING cage and to a minor extent the BAK/C cage, seem to be a good alternative to bone cement in cervical interbody fusion. Other characteristics, such as the effect of implant design on subsidence tendency and the promotion of bone ingrowth, have to be determined in further studies.

Biomechanical evaluation of a new modular rod-screw implant system for posterior instrumentation of the occipito-cervical spine: in-vitro comparison with two established implant systems

Posterior instrumentation of the occipito-cervical spine has become an established procedure in a variety of indications. The use of rod-screw systems improved posterior instrumentation as it allows optimal screw positioning adapted to the individual anatomic situation. However, there are still some drawbacks concerning the different implant designs. Therefore, a new modular rod-screw implant system has been developed to overcome some of the drawbacks of established systems. The aim of this study was to evaluate whether posterior internal fixation of the occipito-cervical spine with the new implant system improves primary biomechanical stability. Three different internal fixation systems were compared in this study: the CerviFix System, the Olerud Cervical Rod Spinal System and the newly developed Neon Occipito Cervical System. Eight human cervical spine CO/C5 specimens were instrumented from C0 to C4 with occipital fixation, transarticular screws in C1/C2 and lateral mass or pedicle screws in C3 and C4. The specimens were tested in flexion/extension, axial rotation, and lateral bending using pure moments of +/- 2.5 Nm without axial preload. After testing the intact spine, the different instrumentations were tested after destabilising C0/C2 and C3/C4. Primary stability was significantly increased, in all load cases, with the new modular implant system compared to the other implant systems. Pedicle screw instrumentation tended to be more stable compared to lateral mass screws; nevertheless, significant differences were observed only for lateral bending. As the experimental design precluded any cyclic testing, the data represent only the primary stability of the implants. In summary, this study showed that posterior instrumentation of the cervical spine using the new Neon Occipito Cervical System improves primary biomechanical stability compared to the CerviFix System and the Olerud Cervical Rod Spinal System.

Vertebral body replacement with a bioglass-polyurethane composite in spine metastases--clinical, radiological and biomechanical results

Metastatic spine lesions frequently require corpectomy in order to achieve decompression of the spinal cord and restoration of spinal stability. A variety of systems have been developed for vertebral body replacement. In patients with prolonged life expectancy due to an improvement of both systemic and local therapy, treatment results can be impaired by a loosening at the implant-bone interface or mechanical failure. Furthermore, early detection of a metastatic recurrence using sensitive imaging modalities like computed tomography (CT) and magnetic resonance imaging (MRI) is possible in these patients without artefact interference. The aim of our pilot study was to evaluate the clinical applicability and results of a new radiolucent system for vertebral body replacement in the lumbar spine. The system consists of bone-integrating biocompatible materials - a polyetherurethane/bioglass composite (PU-C) replacement body and an integrated plate of carbon-fibre reinforced polyetheretherketone (CF-PEEK) - and provides high primary stability with anterior instrumentation alone. In a current prospective study, five patients with metastatic lesions of the lumbar spine were treated by corpectomy and reconstruction using this new system. Good primary stability was achieved in all cases. Follow-up (median 15 months) using CT and MRI revealed progressive osseous integration of the PU-C spacer in four patients surviving more than 6 months. Results obtained from imaging methods were confirmed following autopsy by biomechanical investigation of an explanted device. From these data, it can be concluded that implantation of the new radiolucent system provides sufficient long-term stability for the requirements of selected tumour patients with improved prognosis.

[Mechanical modification of callus healing]

Interfragmentary movement and size of the fracture gap influence fracture healing. Limited movements promote callus formation and may result in increased mechanical stability. Although larger movements still promote callus formation, the bony consolidation of the fracture is hampered. Fracture healing is also hampered if the size of the fracture gap is too large. A combination of large movement and large gap bears the risk of non-union. Therefore, having in mind a minimally invasive surgical approach, one should strive for good reduction of the fracture ends and flexible yet stable osteosynthesis. Dynamization of the fracture by enabling axial movement will close the fracture gap, stimulate tissue differentiation and possibly accelerate the healing process. External mechanical stimulation, however, has not been shown to effectively enhance the healing process under flexible fixation or in load-bearing patients.

A new radiolucent system for vertebral body replacement: its stability in comparison to other systems

Anterior intervention of metastatic lesions of the spine can accomplish relief of pain, spinal decompression, and restoration of spinal stability. Ventral vertebral body replacements have been developed to provide these conditions but there have been problems with loosening at the implant-bone interface, mechanical failure, and X-ray artifacts from the metal. Intraoperative stability of the vertebral body replacement is especially critical to avoid loosening of the implant and to achieve long-term bony incorporation. This study compared the biomechanical performance in vitro of a new radiolucent system for vertebral body replacement to three currently marketed systems. The new system features a composite bioglass-polyurethane body and a new configuration of polymeric fastening hardware. Range of motion, neutral zone, and several interfacial motion parameters were measured under pure moments of 3.75 Nm in the three anatomical directions. The new system provided the significantly highest restraint of motion for all parameters. Mechanically, the new system is preferable at least initially to a sampling of systems representative of those currently used.

Load-displacement properties of the normal and injured lower cervical spine in vitro

The objective of this study was to determine which discoligamentous structures of the lower cervical spine provide significant stability with regard to different loading conditions. Accordingly, the load-displacement properties of the normal and injured lower cervical spine were tested in vitro. Four artificially created stages of increasing discoligamentous instability of the segment C5/6 were compared to the normal C5/6 segment. Six fresh human cadaver spine segments C4-C7 were tested in flexion/extension, axial rotation, and lateral bending using pure moments of +/- 2.5 Nm without axial preload. Five conditions were investigated consecutively: (1) the intact functional spinal unit (FSU) C5/6; (2) the FSU C5/6 with the anterior longitudinal ligament and the intertransverse ligaments sectioned; (3) the FSU C5/6 with an additional 10-mm-deep incision of the anterior half of the anulus fibrosus and the disc; (4) the FSU C5/6 with additionally sectioned ligamenta flava as well as interspinous and supraspinous ligaments; (5) the FSU C5/6 with additional capsulotomy of the facet joints. In flexion/extension, significant differences were observed concerning range of motion (ROM) and neutral zone (NZ) for all four stages of instability compared to the intact FSU. In axial rotation, only the stage 4 instability showed a significantly increased ROM and NZ compared to the intact FSU. For lateral bending, no significant differences were observed. Based on these data, we conclude that flexion/extension is the most sensitive load-direction for the tested discoligamentous instabilities.

Effects of specimen length on the monosegmental motion behavior of the lumbar spine

STUDY DESIGN

An in vitro biomechanical analysis of the segmental motion behavior of the same segments in polysegmental (five segments), bisegmental, and monosegmental specimens using sheep lumbosacral spines.

OBJECTIVES

To investigate the effect of specimen length on monosegmental motion behavior. These data may be helpful in planning in vitro tests and in comparing results of studies using specimens of different lengths.

SUMMARY OF BACKGROUND DATA

The length of spinal specimens used for in vitro stability tests varies greatly, depending on the purpose of the study. Some investigators prefer testing specimens with one adjacent segment on either end of the region of interest. Others favor specimens as short as possible.

METHODS

In a first step, seven sheep spine specimens, L3-S1 (note that sheep spines normally have seven lumbar vertebrae), each were tested without preload in a spine-loading apparatus. Alternating sequences of pure lateral bending, flexion/extension, and axial rotation moments (+/-3.75 Nm) were applied continuously. The motion in each single segment was measured simultaneously. Then, these polysegmental specimens were cut into two bisegmental specimens, L3-L5 and L6-S1, and tested in the same way. Finally, another vertebra was removed to obtain two monosegmental specimens, L3-L4 and L7-S1, and to test them as described.

RESULTS

In general, the range of motion at L3-L4 and L7-S1 was smaller when tested in polysegmental than in monosegmental specimens. In polysegmental specimens (five segments), the range of motion at L3-L4 and L7-S1 was approximately 80% (range, 70.6-92.5%) and in bisegmental specimens approximately 95% (range, 66.7-100%) of their range of motion measured in monosegmental specimens. Neutral zone and coupled motions showed the inverse behavior. Significant differences were found. However, they were not consistent with either the loading direction or with the specimen length.

CONCLUSIONS

For comparison of results, the specimen length should be kept constant within one experiment. Segmental motion behavior of specimens with different lengths should be compared only qualitatively.

[In vitro cell behavior of human osteoblasts after physiological dynamic stretching]

The cell activity of human bone derived cell cultures was studied after mechanical stimulation by cyclic strain at a magnitude occurring in physiologically loaded bone tissue. Monolayers of subconfluently grown human bone derived cells were stretched in rectangular silicone dishes with cyclic uniaxial movement along their longitudinal axes. Strain was applied over two days for 30 min per day with a frequency of 1 Hz and a strain magnitude of 1000 mustrain. Cyclic stretching of the cells resulted in an increased proliferation (10-48%) and carboxyterminal collagen type I propeptide release (7-49%) of human cancellous bone derived osteoblasts while alkaline phosphatase activity and osteocalcin release were significantly reduced by 9-25% and 5-32% respectively. These results demonstrate that cyclic strain at physiologic magnitude leads to an increase of osteoblast activities related to matrix production while those activities which are characteristic for the differentiated osteoblast and relevant for matrix mineralization are decreased.

The influence of stiffness of the fixator on maturation of callus after segmental transport

The treatment of large bony defects by callus distraction is well accepted, but the duration of treatment is long and the rate of complications increases accordingly. We have examined the effect of the stiffness of the axial fixator on reducing the time for maturation of callus.

We created a mid-diaphyseal defect of 15 mm in the metatarsal bone in sheep and stabilised it with a ring fixator. After four days a bony segment was transported for 16 days at 1 mm per day. After 64 days the animals were divided into four groups, three with axial interfragmentary movement (IFM) of 0.5, 1.2 and 3.0 mm, respectively, and a control group.

The 3.0 mm IFM group had the smallest bone density (p = 0.001) and area of callus and the largest IFM after 12 weeks; it also had typical clinical signs of hypertrophic nonunion. The most rapid stiffening of the callus was in the 0.5 mm group which had the smallest IFM (p = 0.04) after 12 weeks and radiological signs of bridging of the defect. These results indicate that suitable dynamic axial stimulation can enhance maturation of distraction callus when the initial amplitude is small, but that a large IFM can lead to delayed union.

Stabilizing effect of posterior lumbar interbody fusion cages before and after cyclic loading

OBJECT

The function of interbody fusion cages is to stabilize spinal segments primarily by distracting them as well as by allowing bone ingrowth and fusion. An important condition for efficient formation of bone tissue is achieving adequate spinal stability. However, the initial stability may be reduced due to repeated movements of the spine during everyday activity. Therefore, in addition to immediate stability, stability after cyclic loading is of remarkable relevance; however, this has not yet been investigated. The object of this study was to investigate the immediate stabilizing effect of three different posterior lumbar interbody fusion cages and to clarify the effect of cyclic loading on the stabilization.

METHODS

Before and directly after implantation of a Zientek, Stryker, or Ray posterior lumbar interbody fusion cage, 24 lumbar spine segment specimens were each evaluated in a spine tester. Pure lateral bending, flexion-extension, and axial rotation moments (+/- 7.5 Nm) were applied continuously. The motion in each specimen was measured simultaneously. The specimens were then loaded cyclically (40,000 cycles, 5 Hz) with an axial compression force ranging from 200 to 1000 N. Finally, they were tested once again in the spine tester.

CONCLUSIONS

In general, a decrease of movement in all loading directions was noted after insertion of the Zientek and Ray cages and an increase of movement after implantation of a Stryker cage. In all three cage groups greater stability was demonstrated in lateral bending and flexion than in extension and axial rotation. Reduced stability during cyclic loading was observed in all three cage groups; however, loss of stability was most pronounced when the Ray cage was used.

Dynamic cell stretching increases human osteoblast proliferation and CICP synthesis but decreases osteocalcin synthesis and alkaline phosphatase activity

The cell activity of human-bone-derived cell cultures was studied after mechanical stimulation by cyclic strain at a magnitude occurring in physiologically loaded bone tissue. Monolayers of subconfluently grown human-bone-derived cells were stretched in rectangular silicone dishes with cyclic predominantly uniaxial movement along their longitudinal axes. Strain was applied over two days for 30 min per day with a frequency of 1 Hz and a strain magnitude of 1000 microstrain. Cyclic stretching of the cells resulted in an increased proliferation (10-48%) and carboxyterminal collagen type I propeptide release (7-49%) of human-cancellous bone-derived osteoblasts while alkaline phosphatase activity and osteocalcin release were significantly reduced by 9-25 and 5-32%, respectively. These results demonstrate that cyclic strain at physiologic magnitude leads to an increase of osteoblast activities related to matrix production while those activities which are characteristic for the differentiated osteoblast and relevant for matrix mineralization are decreased.

The influence of stiffness of the fixator on maturation of callus after segmental transport

The treatment of large bony defects by callus distraction is well accepted, but the duration of treatment is long and the rate of complications increases accordingly. We have examined the effect of the stiffness of the axial fixator on reducing the time for maturation of callus. We created a mid-diaphyseal defect of 15 mm in the metatarsal bone in sheep and stabilised it with a ring fixator. After four days a bony segment was transported for 16 days at 1 mm per day. After 64 days the animals were divided into four groups, three with axial interfragmentary movement (IFM) of 0.5, 1.2 and 3.0 mm, respectively, and a control group. The 3.0 mm IFM group had the smallest bone density (p = 0.001) and area of callus and the largest IFM after 12 weeks; it also had typical clinical signs of hypertrophic nonunion. The most rapid stiffening of the callus was in the 0.5 mm group which had the smallest IFM (p = 0.04) after 12 weeks and radiological signs of bridging of the defect. These results indicate that suitable dynamic axial stimulation can enhance maturation of distraction callus when the initial amplitude is small, but that a large IFM can lead to delayed union.