Matti Scholz

Comparison between sheep and human cervical spines: An anatomic, radiographic, bone mineral density, and biomechanical study

Study Design. The quantitative anatomic, radiographic, computerized tomographic, and biomechanical data of sheep and human cervical spines were evaluated. Objectives. To compare the anatomic, radiographic, computerized tomographic, and biomechanical data of human and sheep cervical spines to determine whether the sheep spine is a suitable model for human spine research. Summary of Background Data. Sheep spines have been used in several in vivo and in vitro experiments. Quantitative data of the normal sheep cervical spine are lacking, yet these data are crucial to discussion about the results of such animal studies. Methods. In this study, 20 fresh adult female Merino sheep cervical spines and 20 fresh human cadaver cervical spines were evaluated anatomically, radiographically, computerized tomographically, and biomechanically. Three linear and two angular parameters were evaluated on four digital radiographic views: anteroposterior, right lateral in neutral position, flexion, and extension. Quantitative computed tomography scans at the center of each vertebral body and 3 mm below both endplates were analyzed for bone mineral density measurements. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending by a nondestructive stiffness method using a nonconstrained testing apparatus. Range of motion and stiffness of each motion segment were calculated. Additionally, 10 linear anatomic parameters of each vertebra were measured using a digital ruler. Results. Anterior and mean disc space height in the sheep cervical spine increased constantly from C2–C3 to C6–C7, whereas middle disc space height decreased and posterior disc space height remained unchanged. Anterior and mean disc space height were significantly higher in sheep. In both sheep and human cervical spines, intervertebral angles were not significantly different. Standard deviations of bone mineral density in the human cervical spine were fourfold higher than in the sheep cervical spine, yet no significant differences were found in bone mineral density values between the two species. Range of motion differed significantly between the two species except in flexion–extension of C3–C4, C5–C6, axial rotation of C2–C3, and lateral bending of C2–C3, C3–C4, and C4–C5. Stiffness also was significantly different except in flexion–extension of C2–C3, C4–C5, C5–C6, and lateral bending of C2–C3, C3–C4, and C4–C5. Anatomic evaluation showed no difference in upper endplate parameters for C4 and C5. Conclusions. Although several differences were found between human and sheep cervical spines, the small intergroup standard deviations and the good comparability with the human spine encourage the use of the sheep cervical spine as a model for cervical spine research. On the basis of the quantitative data obtained in this study, the sheep motion segment C3–C4 seemed to be the most reliable model for the corresponding human motion segment.

Biomechanical Comparison of Expandable Cages for Vertebral Body Replacement in the Thoracolumbar Spine

Study Design. An in vitro biomechanical study of expandable cages for vertebral body replacement in the human thoracolumbar spine. Objectives: The purpose of this study was to compare the in vitro biomechanical properties of 3 different expandable cages with a nonexpandable cage. Summary and Background Data. Recently, there has been a rapid increase in the use and the commercial availability of expandable cages for vertebral body replacement in the thoracolumbar spine. Although all 3 expandable cages, evaluated in this study, are approved for clinical use in Europe, little information is available concerning the biomechanical properties of these implants. Material and Methods. Thirty-two human thoracolumbar spines (T11 to L3) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive loading technique using an unconstrained testing apparatus. Three-dimensional displacement was measured using an optical measurement system. First, all motion segments were tested intact. After complete corporectomy of L1, cages were implanted according to producer’s information. The following implants (n = 8/group) were tested: 1) meshed titanium cage (nonexpandable cage, DePuy AcroMed); 2) X-tenz (expandable cage, DePuy AcroMed); 3) Synex (expandable Cage; Synthes); and 4) VBR (expandable cage, Ulrich). Finally, posterior stabilization using the Universal Spine System (Synthes), posterior-anterior stabilization using the Universal Spine System (Synthes), and anterior plating (Locking Compression Plate, Synthes) was applied to each test specimen. The mean apparent stiffness values, range of motion, and neutral and elastic zone were calculated from the corresponding load-displacement curves. Results. No significant differences could be determined between the in vitro biomechanical properties of expandable and nonexpandable cages. In comparison to the intact motion segment, isolated anterior stabilizationusing cages and anterior plating significantly decreased stiffness and increased range of motion in all directions. In contrast, additional posterior stabilization significantly increased stiffness and decreased range of motion in all directions compared to the intact motion segment. The combined anterior-posterior stabilization demonstrated greatest stiffness results. Conclusion. Biomechanical results indicate that design variations of expandable cages for vertebral body replacement are of little importance. Additionally, no significant difference could be determined between the biomechanical properties of expandable and nonexpandable cages. After corporectomy, isolated implantation of expandable cages plus anterior plating was not able to restore normal stability of the motion segment. Therefore, isolated anterior stabilization using cages plus Locking Compression Plate should not be used for vertebral body replacement in the thoracolumbar spine.

A New Stand-alone Anterior Lumbar Interbody Fusion Device: Biomechanical Comparison with Established Fixation Techniques

Study Design. Established lumbar fixation methods were assessed biomechanically, and a comparison was made with a new stand-alone anterior lumbar interbody cage device incorporating integrated anterior fixation. Objectives. To compare the stability of a new stand-alone anterior implant (Test-device) with established fixation methods to assess its suitability for clinical use. Our hypothesis being that the Test-device would provide stability comparable to that provided by an anterior cage when supplemented with posterior pedicle screw fixation. Summary of Background Data. It is accepted that the use of rigid pedicle screw instrumentation increases the chance of achieving a solid fusion, but its use may be associated with a significant increase in postoperative morbidity caused by disruption of the posterior musculature. It is also evident that this increased fusion rate is generally not associated with increased clinical success. This dilemma has led to a search for a solution and to the development of the Test-device anterior lumbar interbody device. Methods. The kinematic properties of either the L3–L4 or L4–L5 lumbar motion segment of 8 cadaveric lumbar spines have been tested using the following sequence of fixation: intact, Test-device, Test-device and translaminar facet screws (TS), Cage and TS, Cage and Universal Spine System (USS), and Cage and small stature USS. Results. All fixation techniques except the cage and TS decreased (P < 0.05) range of motion (ROM), neutral zone (NZ), and elastic zone (EZ), and increased (P < 0.05) stiffness in comparison to the intact motion segment in all test modes. There was a significant increase (P < 0.01) in the ROM, NZ, and EZ, and decrease in the stiffness of the cage and TS group in comparison to all other stabilizationtechniques in flexion and rotation. There was no significant difference in the ROM, NZ, EZ, and stiffness between the Test-device and cage and USS groups in flexion, extension, and bending. The Test-device resulted in a significantly lower EZ (P < 0.05) and a significantly higher stiffness (P < 0.05) in rotation than all other fixation methods. Conclusions. The Test-device alone provided similar and the Test-device and TS higher stability than the pedicle screw constructs evaluated. These results support progression to clinical trials using the Test-device as a stand-alone implant.

A new stand-alone cervical anterior interbody fusion device: biomechanical comparison with established anterior cervical fixation devices.

Study Design. A new anchored spacer—a low-profile cervical interbody fusion cage with integrated anterior fixation—was compared biomechanically to established anterior cervical devices. Objective. To evaluate the fixation properties of the new stand-alone device and compare these properties with established fixation methods. The hypothesis is that the new device will provide stability comparable to that provided by an anterior cervical cage when supplemented with an anterior plate. Summary of Background Data. It is accepted that the use of anterior cervical plating increases the chance of achieving a solid fusion. However, its use may be associated with an increase in operation time and a higher postoperative morbidity caused by a larger anterior approach and disruption of the anterior musculature. This dilemma has led to the development of a new, low profile stand-alone cervical anterior cage device with integrated screw fixation. Methods. Twenty-four human cadaveric C4–C7 cervical spines were loaded nondestructively with pure moments in a nonconstraining testing apparatus to induce flexion, extension, lateral bending, and axial rotation while angular motion was measured optoelectronically. The specimens were tested:1. Intact (N = 24).2. After discectomy and anterior stabilization.a. Interbody cage + locking plate (N = 8).b. Interbody cage + dynamic plate (N = 8).c. Anchored spacer (N = 8).3. After ventral plate removal of group 2a and 2b (N = 16). Results. All fixation techniques decreased range of motion (ROM) and lax zone (LZ) (P < 0.05) in all test modes compared with the intact motion segment and cage-only group. There were no significant differences between the anchored spacer and cage + locking plate or cage + dynamic plate. Conclusion. The anchored spacer provided a similar biomechanical stability to that of the established anterior fusion technique using an anterior plate plus cage and has a potentially lower perioperative and postoperative morbidity. These results support progression to clinical trials using the cervical anchored spacer as a stand-alone implant.

Biomechanical testing of the lumbar facet interference screw

Study Design. An in vitro study was conducted to determine the biomechanical properties of a new simple, percutaneous, posterior fixation technique for the lumbar spine involving a new implant, the so-called Lumbar Facet Interference Screw. Objectives. The purpose of this study was to compare the biomechanical properties of this new fixation device with translaminar and pedicle screw fixation. Summary of Background Data. Several techniques were described to perform a minimal invasive posterior stabilization of the lumbar spine after an anterior lumbar interbody fusion procedure. Yet, due to the high complexity of these minimally invasive surgical procedures, currently, hardly any of these percutaneous posterior fixation techniques is carried out routinely. Methods. Ten human lumbar spines were tested in flexion, extension, axial rotation, and lateral bending using a nonconstrained testing method. First, all motion segments were evaluated intact (group 1). After complete discectomy of L4–L5, the following stabilization techniques were tested sequentially (n = 10/group): group 2: “stand-alone” cage; group 3: cage plus translaminar screws; group 4: cage plus Lumbar Facet Interference Screw; and group 5: cage plus pedicle screws. Stiffness, ranges of motion, and neutral and elastic zones were determined. Results. In comparison to the intact motion segment, the “stand-alone” cage showed a significantly higher (P < 0.05) range of motion, neutral zone, and elastic zone and a significantly lower (P < 0.05) stiffness in extension and rotation. Generally, all fixation techniques using cages plus posterior stabilization decreased range of motion, neutral zone, and elastic zone and increased stiffness in comparison to the “stand-alone” cage group. There was no significant difference between the cage plus interference screw and the cage plus translaminar screw group in all test modes. In comparison to the 2 facet joint stabilization techniques, pedicle screw stabilization decreased (P < 0.01) range of motion, neutral zone, and elastic zone and increased (P < 0.01) stiffness significantly in flexion and rotation. Conclusions. Results of this study indicate that the new Lumbar Facet Interference Screw fixation yields initial biomechanical stability similar to translaminar screw fixation, yet inferior biomechanical stability compared to pedicle screw fixation. Although these results are encouraging, additional biomechanical studies including cyclic loading tests have to evaluate the mid- and long-term stabilization capacity of this new minimally invasive fixation technique before human application.

Bioabsorbable interbody cages in a sheep cervical spine fusion model

Study Design. An experimental study using a sheep cervical spine interbody fusion model. Objectives. To compare interbody fusion of an autologous tricortical iliac crest bone graft with two bioabsorbable cages and to determine whether there are differences between the three interbody fusion techniques in 1) the ability to preserve postoperative distraction, 2) the biomechanical stability, and 3) the histologic characteristics of intervertebral bone matrix formation. Summary and Background Data. Bioabsorbable cages would be beneficial compared with metallic cages; however, currently no suitable bioabsorbable interbody fusion cage is available. Method. Twenty-four sheep underwent C3/C4 discectomy and fusion. The following stabilization techniques were used: Group 1) autologous tricortical iliac crest bone graft (n = 8); Group 2) bioabsorbable cage made of 70/30 poly(l-lactide-co-d,l-lactide) (experimental) filled with autologous cancellous bone graft (n = 8); Group 3) bioabsorbable cage made of a polymer-calciumphosphate composite (Biomet Europe, Dordrecht, The Netherlands) filled with autologous cancellous bone graft (n = 8). Radiographic scans to determine disc space height were performed before and after surgery and after 1, 2, 4, 8, and 12 weeks, respectively. After 12 weeks, animals were killed, and fusion sites were evaluated using functional radiographic views in flexion and extension. Quantitative computed tomographic scans were used to assess bone mineral density, bone mineral content, and bony callus volume. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending to determine stiffness, ROM, and neutral and elastic zone. Histomorphological and histomorphometrical analysis were performed to evaluate fusion and foreign body reactions associated with the bioabsorbable cages. Results. Over a 12-week period, the polymer-calciumphosphate composite cage showed significantly higher values for disc space height compared with the bone graft and the poly(l-lactide-co-d,l-lactide) cage. Additionally, the polymer-calciumphosphate composite cage demonstrateda significantly higher stiffness and lower ROM, neutral zone, and elastic zone in axial rotation and lateral bending than any other group. However, quantitative computed tomographic scans demonstrated cracks in six of the eight polymer-calciumphosphate composite cages after 12 weeks. Histologically, the highest bone volume/total volume ratio and the highest fusion rate were found in the polymer-calciumphosphate composite cage group. Although the poly(l-lactide-co-d,l-lactide) cage showed grade I through III foreign body reactions in all fusion areas, only two animals developed grade I foreign body reactions with the polymer-calciumphosphate composite cage. Conclusion. After 12 weeks, there was no significant difference between the bioabsorbable poly(l-lactide-co-d,l-lactide) cage and the tricortical bone graft. In comparison to the tricortical bone graft, the bioabsorbable polymer-calciumphosphate composite cage showed significantly better distractive properties, a significantly higher biomechanical stiffness, and an advanced interbody fusion; however, six of eight polymer-calciumphosphate composite cages cracked. Although the fate of the foreign body reactions and the cracks is currently unclear for both bioabsorbable cages, the early appearance of large osteolysis associated with use of the poly(l-lactide-co-d,l-lactide) cage allows skepticism regarding the value of this bioabsorbable implant.

Biomechanical evaluation of different asymmetrical posterior stabilization methods for minimally invasive transforaminal lumbar interbody fusion.

OBJECT Beside several other advantages, the transforaminal approach for lumbar interbody fusion offers the possibility of reducing surgical trauma by limiting the approach to only 1 side. This requires posterior stabilization methods, which are applied without the need to damage contralateral muscles and soft tissues. The goal in this study was to compare different posterior stabilization methods for minimally invasive transforaminal lumbar interbody fusion (TLIF) biomechanically. METHODS Stiffness testing was performed in 8 fresh-frozen human cadaveric lumbar spine motion segments, including the following sequentially tested configurations: 1) native motion segment; 2) TLIF and bilateral pedicle screw (PS) construct; 3) TLIF and ipsilateral PS construct; 4) TLIF and ipsilateral PSs plus contralateral translaminar facet screws according to the Magerl technique; and 5) TLIF and ipsilateral PSs plus contralateral lumbar facet interference screw (LFIS). RESULTS In extension, the unilateral range of motion (uROM) and elastic zone (EZ) were significantly lower than native motion segments for bilateral PS and LFIS. There were no significant differences among the different stabilization methods. In flexion, uROM and EZ were significantly lower than the native segment in the spines treated with bilateral PSs and translaminar facet screws. The LFIS differed from the native segment in EZ only. Again, there were no significant differences between the different posterior stabilization methods. In lateral bending, the EZ of spines treated with uni- and bilateral PS differed significantly. There were no additional significant differences. In rotation, the stiffness values of bilateral PS were significantly higher than native, unilateral PS, and LFIS. The comparison between ipsi- and bilateral PS showed a tendency, but not a significant difference for uROM and EZ. There was no statistically significant evidence that the TLIF method led to an asymmetrical motion behavior in our study. CONCLUSIONS Bilateral PS augmentation offers significantly more stability than unilateral PSs in the majority of the test modes. There was no significant difference between the other tested methods. All tested stabilization methods could achieve at least the stability of the native segment.

Therapieempfehlungen zur Versorgung von Verletzungen der Brust- und Lendenwirbelsäule

ZusammenfassungIn dieser Arbeit werden Empfehlungen zur Versorgung von Frakturen der Brust- (BWS) und der Lendenwirbelsäule (LWS) gegeben. Die Empfehlungen beruhen auf der Erfahrung der beteiligten Wirbelsäulenchirurgen der Arbeitsgemeinschaft „Wirbelsäule“ der Deutschen Gesellschaft für Unfallchirurgie unter Berücksichtigung der aktuellen Literatur. Grundlagen der Diagnostik, der konservativen und operativen Therapie werden dargestellt. Die Frakturen werden Anhand von morphologischen Kriterien, wie der Zerstörung des Wirbelkörpers, der Fragmentdislokation, der Einengung des Spinalkanals, der Achsabweichungen und der Abweichung vom individuellen sagittalen Profil beurteilt. Die Abweichung von dem individuellen sagittalen Profil wird anhand der Änderung des monosegmentalen oder bisegmentalen Grund-Deckplatten-Winkels bestimmt. Es werden die Therapieoptionen für den knochengesunden Patienten aufgezeigt.AbstractThis paper gives recommendations for treatment of thoracolumbar and lumbar spine injuries. The recommendations are based on the experience of the involved spine surgeons, who are part of a study group of the “Deutsche Gesellschaft für Unfallchirurgie” and a review of the current literature. Basics of diagnostic, conservative, and operative therapy are demonstrated. Fractures are evaluated by using morphologic criteria like destruction of the vertebral body, fragment dislocation, narrowing of the spinal canal, and deviation from the individual physiologic profile. Deviations from the individual sagittal profile are described by using the monosegmental or bisegmental end plate angle. The recommendations are developed for acute traumatic fractures in patients without severe osteoporotic disease.

Biomechanical comparison of bioabsorbable cervical spine interbody fusion cages

Study Design. In vitro biomechanical study of bioabsorbable cervical spine interbody fusion cages using a sheep model. Objectives: The purpose of this study was to evaluate the segmental stability provided by 2 new developed bioabsorbable cervical spine interbody fusion cages and to compare it with a tricortical iliac crest bone graft and a titanium meshed interbody fusion cage. Further, the biomechanical effect of an additional anterior plate instrumentation was determined. Summary and Background Data. Despite the initial favorable results, the long-term effects of metallic cage devices on spinal motion segments are still unknown. Furthermore, shortcomings of metallic cages like migration, adjacent level degeneration, stenotic myelopathy, and artifacts in postoperative radiologic assessment have already been reported. Bioabsorbable cages have been designed to avoid these complications. Currently, no information is available about the biomechanical properties of bioabsorbable cervical spine interbody fusion cages. Methods. Forty sheep cervical spines (C2–C5) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive stiffness method using a nonconstrained testing apparatus. First, the motion segment C3–C4 was tested intact. After complete discectomy, the following groups were evaluated: autologous iliac crest bone graft, titanium mesh cylinder (Harms, DePuy AcroMed), bioabsorbable PDLLA-cage (experimental), and bioabsorbable Resorbon cage (Biomet Merck). Further, all implants were tested with an additional anterior plate instrumentation. The mean apparent stiffness, range of motion, neutral zone, and elastic zone were calculated from the corresponding load-displacement curves. Results. No significant difference in range of motion and segmental stiffness among the tricortical iliac crest bone graft, meshed titanium Harms cage, and PDLLA-cage could be determined. The Resorbon cage significantly (P < 0.05) decreased range of motion and increased stiffness in rotation and flexion in comparisonto all tested implants and the intact motion segment. An additional anterior plate significantly (P < 0.05) decreased range of motion and increased stiffness in flexion and extension. Conclusion. In this study, bioabsorbable cages demonstrated biomechanical in vitro properties equal or superior to metallic cages. From the biomechanical point of view, bioabsorbable cages, especially the Resorbon cage, may be a viable alternative to current metallic interbody cage devices. However, animal experimental in vivo evaluation of bioabsorbable cervical spine interbody fusion cages still has to be performed.

Recommendations for the treatment of thoracolumbar and lumbar spine injuries

ZusammenfassungIn dieser Arbeit werden Empfehlungen zur Versorgung von Frakturen der Brust- (BWS) und der Lendenwirbelsäule (LWS) gegeben. Die Empfehlungen beruhen auf der Erfahrung der beteiligten Wirbelsäulenchirurgen der Arbeitsgemeinschaft „Wirbelsäule“ der Deutschen Gesellschaft für Unfallchirurgie unter Berücksichtigung der aktuellen Literatur. Grundlagen der Diagnostik, der konservativen und operativen Therapie werden dargestellt. Die Frakturen werden Anhand von morphologischen Kriterien, wie der Zerstörung des Wirbelkörpers, der Fragmentdislokation, der Einengung des Spinalkanals, der Achsabweichungen und der Abweichung vom individuellen sagittalen Profil beurteilt. Die Abweichung von dem individuellen sagittalen Profil wird anhand der Änderung des monosegmentalen oder bisegmentalen Grund-Deckplatten-Winkels bestimmt. Es werden die Therapieoptionen für den knochengesunden Patienten aufgezeigt.AbstractThis paper gives recommendations for treatment of thoracolumbar and lumbar spine injuries. The recommendations are based on the experience of the involved spine surgeons, who are part of a study group of the “Deutsche Gesellschaft für Unfallchirurgie” and a review of the current literature. Basics of diagnostic, conservative, and operative therapy are demonstrated. Fractures are evaluated by using morphologic criteria like destruction of the vertebral body, fragment dislocation, narrowing of the spinal canal, and deviation from the individual physiologic profile. Deviations from the individual sagittal profile are described by using the monosegmental or bisegmental end plate angle. The recommendations are developed for acute traumatic fractures in patients without severe osteoporotic disease.