Reconstruction of the lateral ligaments: do the anatomical procedures restore physiologic ankle kinematics?

BACKGROUND: If conservative therapy fails, the standard treatment for chronic ankle instability is surgical reconstruction of the lateral ligaments. For the last seventy years, the tenodesis principles have been used for reconstruction. Recently however, surgical reconstructions--respecting the intact joint anatomy--have been developed, thus called "anatomical reconstruction principles". METHODS: This study focused on the investigation of the range of motion of the ankle and the subtalar joint following anatomical reconstruction surgery. Three different types of anatomical reconstruction procedures were compared: Direct ligament repair, tendon graft and carbon-fiber implant. RESULTS: All procedures restored the original range of motion of the subtalar joint, except for the plantarflexed/dorsiflexed positions. As for the talocrural joint, the tendon graft and the carbon fiber implant left a small laxity for movements of inversion/eversion and internal/external rotation. The direct repair procedure achieved a more accurate result and restored the physiologic kinematics almost completely. During each procedure the insertion points and the direction of the original ligaments were maintained. However, the different results for the procedure of direct ligament repair compared to the other two anatomical reconstruction procedures showed that this condition alone is not sufficient to perfectly restore the kinematics of the talocrural and subtalar joints. It is important to note that none of the procedures caused a restriction of the range of motion. CONCLUSIONS: The maintenance of the range of hindfoot motion decreases the risk of osteoarthritis as well as chronic pain or problems for the patient to walk on uneven surface. Therefore, we believe that standard therapy for chronic instability of the ankle should include direct surgical reconstruction of the ligaments. If this direct procedure cannot be performed because of poor quality of the ligaments an alternative anatomical reconstruction procedure should be considered.

ISSLS prize winner: A novel approach to determine trunk muscle forces during flexion and extension: a comparison of data from an in vitro experiment and in vivo measurements.

STUDY DESIGN: Disc pressure and fixator load were measured in an in vitro setup and compared to in vivo measurements with the identical transducers from the two groups participating in this study. OBJECTIVES: The goal of this in vitro study was to determine the magnitude of trunk muscle forces during flexion and extension. The loading conditions in this study accounted for body weight, local and global muscles, and forces resulting from the support of the abdominal soft tissue in different postures. Resulting intersegmental motions and intradiscal pressure in each segment and the six load components in both rods of an internal fixator were determined. SUMMARY OF BACKGROUND DATA: The spine is primarily stabilized by muscle forces, which greatly influence spinal loads. However, little information exists on the magnitudes of trunk muscle forces during postures like flexion and extension of the upper body. METHODS: Seven human cadaveric lumbar spines were mounted in a spine tester and adjusted to different degrees of flexion and extension of the upper body with different hip flexions. For each specimen, a total of 124 load cases were studied. They included combinations of a vertical compressive load, a follower load and forces pulling with cables at a plate fixed at the cranial end of the specimen to simulate rectus abdominis, erector spinae, and a supporting force of the abdomen. The muscle forces were varied until the external moment, necessary to keep the lumbar spine specimen in the examined posture, was zero. This was achieved with different muscle force combinations. Loads on internal fixators as well as intradiscal pressure and intersegmental rotation at all levels were measured. The muscle force combination that caused intradiscal pressures and loads in the internal fixator closest to those measured in vivo were assumed to be the muscle forces which can be expected in vivo. RESULTS: Generally, intradiscal pressure was closer to in vivo measurements than the fixator loads. The force in the m. erector spinae increased with the flexion angle but was only slightly influenced by extension. The estimated forces in the erector spinae were 100 N for standing, 130 N for 15 degrees extension, and 520 N for 30 degrees flexion of the upper body. Little influence was found on the intersegmental motion. CONCLUSION: In vitro loading conditions can be approximated closely to in vivo conditions with the simulation of an axial preload, local, and global muscles. This novel approach can help to estimate muscle forces, which can usually not be measured. The results from this study provide important input for FEM models, which may then allow the investigation of different load cases.

[Effect of artificial disk nucleus implant on mobility and intervertebral disk high of an L4/5 segment after nucleotomy]. / Effekt eines künstlichen Nukleusersatzimplantats auf die Beweglichkeit und Bandscheibenhöhe an einem L4/5-Segment nach Nukleotomie.

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.

[Neon--a new angle-stable implant system for dorsal occipitocervical instrumentation. Biomechanical comparison with established systems]. / Neon--ein neues winkelstabiles Implantatsystem für die dorsale okzipitozervikale Instrumentierung. Biomechanischer Vergleich mit etablierten Systemen.

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.

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.

Effect of a prosthetic disc nucleus on the mobility and disc height of the L4-5 intervertebral disc postnucleotomy.

OBJECT: Current procedures for treatment of degenerative disc disease may not restore flexibility or disc height to the intervertebral disc. Recently, a prosthetic device, intended to replace the degenerated nucleus pulposus, was developed. In this biomechanical in vitro test the authors study the effect of implanting a prosthetic nucleus in cadaveric lumbar intervertebral discs postnucleotomy and determine if the flexibility and disc height of the L4-5 motion segment is restored. METHODS: The prosthetic disc nucleus device consists of two hydrogel pellets, each enclosed in a woven polyethylene jacket. Six human cadaveric lumbar motion segments (obtained in individuals who, at the time of death, were a mean age of 56.7 years) were loaded with moments of +/- 7.5 Nm in flexion-extension, lateral bending, and axial rotation. The following states were investigated: intact, postnucleotomy, and after device implantation. Range of motion (ROM) and neutral zone (NZ) measurements were determined. Change in disc height from the intact state was measured after nucleotomy and device implantation, with and without a 200-N preload. CONCLUSIONS: Compared with the intact state (100%), the nucleotomy increased the ROM in flexion-extension to 118%, lateral bending to 112%, and axial rotation to 121%; once the device was implanted the ROM was reduced to 102%, 88%, and 90%, respectively. The NZ increased the ROM to 210%, lateral bending to 173%, and axial rotation to 107% after nucleotomy, and 146%, 149%, 44%, respectively, after device implantation. A 200-N preload reduced the intact and postnucleotomy disc heights by approximately 1 mm and 2 mm, respectively. The original intact disc height was restored after implantation of the device. The results of the cadaveric L4-5 flexibility testing indicate that the device can potentially restore ROM, NZ, and disc height to the denucleated segment.

Effect of an internal fixator and a bone graft on intersegmental spinal motion and intradiscal pressure in the adjacent regions.

Stabilizing a lumbar spine with an implant alters the mechanical properties of the bridged region. In order to determine whether this procedure is associated with higher loads in the adjacent segments, seven lumbar cadaver spines were mounted in a spine tester and loaded with pure moments of flexion/extension, left and right lateral bending, and left and right axial rotation. The material studied comprised intact lumbar spines, intact spines with bisegmental internal spinal fixators, and postcorpectomy spines both with a graft and fixators and with fixators alone. Intradiscal pressures and intersegmental motion were measured at all levels. In the bridged region, these parameters were strongly affected by an internal fixator. In most cases, the effect was small in the regions above and below the fixators. Highly significant differences in these regions (P<0.01) were far below the interspecimen range. We did not find any case where both intradiscal pressure changes and intersegmental motion showed highly significantly differences in the regions adjacent to the bridged one. Our results suggest that disc degeneration, which is sometimes found at the level directly above and below the fixators, is not caused by mechanical factors.

Is it possible to simulate physiologic loading conditions by applying pure moments? A comparison of in vivo and in vitro load components in an internal fixator.

STUDY DESIGN: Loads acting in an internal fixator measured in vitro under the application of pure moments such as those commonly used for implant testing and basic research were compared with loads measured in 10 patients in vivo. OBJECTIVES: To investigate whether these recommended loading conditions are valid by comparing in vivo measurements and those obtained in an in vitro experiment. SUMMARY OF BACKGROUND DATA: Pure bending moments are often preferred as loading conditions for spinal in vitro testing, either for implant testing or basic research. The advantage of this loading pattern is that the bending moment is uniform along the multisegmental specimen. However, functional loading of the spine by muscles or external loads subjects the spine to a combination of forces and moments. METHODS: In an in vivo experiment, loads acting on an internal spinal fixator in 10 patients were determined before and after anterior interbody fusion during flexion, extension, left and right lateral bending, and left and right axial twisting of the upper body with the patient standing. For comparison, an equivalent in vitro data set was created with 7 human lumbar specimens in which the same type of fixator was used. All specimens were tested under the application of pure bending moments in the three main motion planes in the intact state with fixator, after corpectomy, and with bone graft. RESULTS: Consistent qualitative agreement between in vivo and in vitro measurements for the loads acting in the internal spinal fixator were found for axial rotation and lateral bending. For flexion and extension, reasonable agreement was found only for the intact spines with fixators. After corpectomy and after inserting a bone graft, the median values for axial force and bending moment in the sagittal plane in vitro did not agree with in vivo measurements. An axial preload in the in vitro experiment slightly increased the axial compression force and flexion bending moment in the fixators. CONCLUSIONS: The application of pure moments to intact lumbar spinal specimens in vitro produces forces and moments in implants comparable with loads observed in vivo. During basic research on intact specimens or implant testing involving a removed disc or corpectomy, muscle forces are necessary to simulate realistic conditions.

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.

Computer-assisted surgery in posterior instrumentation of the cervical spine: an in-vitro feasibility study.

Transarticular C1/2 screws are widely used in posterior cervical spine instrumentation. The use of pedicle screws in the cervical spine remains uncommon. Due to superior biomechanical stability compared to lateral mass screws, pedicle screws can be used, especially for patients with poor bone quality or defects in the anterior column. Nevertheless there are potential risks of iatrogenic damage to the spinal cord, nerve roots or the vertebral artery associated with both posterior cervical spine instrumentation techniques. Therefore, the aim of this study was to evaluate whether C1/2 transarticular screws as well as transpedicular screws in C3 and C4 can be applied safely and with high accuracy using a computer-assisted surgery (CAS) system. We used 13 human cadaver C0-C5 spine segments. We installed 1.4-mm Kirschner wires transarticular in C 1/2, using a specially designed guide, and drilled 2.5-mm pedicle holes in C3 and C4 with the assistance of the CAS system. Hole positions were evaluated by palpation, CT and dissection. Forty-eight (92%) of the 52 drilled pedicles were correctly positioned after palpation, imaging and dissection. The vertebral artery was not injured in any specimen. All of the 26 C1/2 Kirschner wires were placed properly after imaging and dissection evaluations. No injury to vascular or bony structures was observed. C /2 transarticular screws as well as transpedicular screws in the cervical spine can be applied safely and with high accuracy using a CAS system in vitro. Therefore, this technique may be used in a clinical setting, as it offers improved accuracy and reduced radiation dose for the patient and the medical staff. Nevertheless, users should take note of known sources of possible faults causing inaccuracies in order to prevent iatrogenic damage. Small pedicles, with a diameter of less than 4.0 mm, may not be suitable for pedicle screws.