Effect of lumbar interbody cage geometry on construct stability: a cadaveric study

STUDY DESIGN

Biomechanical study to investigate three-dimensional motion behavior of cadaveric spines in various surgical simulations.

OBJECTIVES

To determine the effect of cage geometry on the construct stability.

SUMMARY OF BACKGROUND DATA

There is a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, and rectangular with and without curvature. However, the effectiveness of cages with different designs and materials to stabilize a decompressed intervertebral space has not been fully studied.

METHODS

Six fresh ligamentous lumbar spine specimens (L1-S2) were subjected to pure moments in the six loading directions. The resulting spatial orientations of the vertebrae were recorded using Optotrak Motion Measurement System. Measurements were made sequentially for intact, bilateral spacer placements across L4-L5 using a posterior approach, supplemented with pedicle screw-rod system fixation, and after the cyclic loading in flexion-extension mode.

RESULTS

The stability tended to decrease after the bilateral cage placement as compared with the intact for all loading cases except flexion. In flexion, the angular displacement decreased to 80% of the intact. However, there was no significant statistical difference seen in stability between intact and after bilateral spacer placement. Following the addition of posterior fixation using pedicle screw-rod system, the stability significantly increased in all directions. Cyclic loading did not have any significant effect on the stability.

CONCLUSIONS

Stand-alone cages restore motion to near-intact levels at best, and supplement instrumentation is essential for significantly increasing the stability of the decompressed segment. The effects of cage geometry and Young's modulus of the cage material do not seem to influence the stability, as compared with the other cagedesigns, especially after supplemental fixation with a posterior system.

Morphological variations of the ulnar styloid process

P.A. radiographs of both wrists were taken in 400 normal individuals to study the configuration of the styloid process of the ulna. Five different morphological variations were noticed, the commonest being an elongated process (102 wrists). Medially deviated (41 wrists), parrot beaked (27 wrists) and hypertrophic (16 wrists) patterns were less common. One person had bilateral unfused separate ossification centres for the ulnar styloid. There was no correlation between the length of the styloid process and ulnar variance.

Anterior iliac crest bone graft. Anatomic considerations

STUDY DESIGN

A morphologic study of the anterior part of the iliac crest was performed.

OBJECTIVE

To define the anatomic characteristics of the anterior part of the ilium and to determine an optimal area to harvest the iliac bone graft from the anterior iliac crest.

SUMMARY OF BACKGROUND DATA

Stress fracture or avulsion fracture of the anterior cut for anterior iliac crest graft have been noted previously. However, there is insufficient published information on the morphology of the anterior part of the ilium relative to the optimal location of harvesting the bone graft.

METHODS

Direct measurements using digital calipers were taken from 30 dried human pelves and 10 cadaveric pelves. The thickness of the anterior part of the ilium was measured, with different starting points on the iliac crest. The length of the bicortical iliac bone graft also was determined.

RESULTS

The thickest portion of the ilium was 18.9 +/- 2.3 mm at the iliac tubercle, which was 45% thicker than at a point 3 cm posterior to the anterior superior iliac spine. The thick region of the anterior iliac crest extended 54.0 +/- 10.2 mm posteriorly from a point 3 cm posterior to the anterior superior iliac spine. The mean length of a 10 mm thick bicortical iliac tubercle bone graft was 36.8 +/- 8.7 mm.

CONCLUSIONS

The region around the iliac tubercle is suitable for harvesting bicortical or tricortical bone graft.

Ulnar translocation after excision of giant cell tumour of distal radius

Translocation of the ipsilateral ulna has been carried out in six cases after radical resection of a giant cell tumour of the distal radius. The carpo-ulnar junction united readily in all cases. Union of the radio-ulnar junction is often delayed, but this can be overcome by the addition of cancellous grafts at the time of ulnar translocation. All the patients had very good forearm rotation and an acceptable appearance. There were no significant complications.

Anterior tibial artery and its actual projection on the lateral aspect of the tibia: a cadaveric study

The anterior tibial artery (ATA) is at risk of injury during high tibial osteotomy, Ilizarov wire placement, pin placement in external fixation, or proximal locking screw insertion, as the artery is not visualized intraoperatively. The ATA is anchored to the oval foramen of the interosseous membrane on the proximal tibia by the deep fascia and recurrent genicular vascular branches. Segment 1 (from the bifurcation of the popliteal artery to the level of the interosseous foramen) and the proximal part of segment 2 (from the interosseous foramen to the level where the artery crosses the anterior border of the tibia) may be damaged when pin, wire or screw placement is directed posterolaterally at that level. Distally, a straight mediolateral pin or Ilizarov wires may lacerate the artery. Segment 2 of the ATA descends against the interosseous membrane in its proximal part, which is projected on the posterior third of the tibia relative to the sagittal plane; in its middle part, it runs close to the lateral cortex of the tibia, it is projected on the middle third of the tibia; in its distal part it runs gradually towards the anterior third of the tibia and contacts with the anterior third of the tibial cortical surface. This information may help reduce risk of injury to the ATA during high tibial osteotomy, external fixation and pin placement or insertion of locking screws.

Intradiscal electrothermal therapy: a treatment option in patients with internal disc disruption

STUDY DESIGN

A literature review was conducted.

OBJECTIVES

To review the anatomy, pathophysiology, diagnosis, procedure, and clinical results of intradiscal electrothermal therapy (IDET).

SUMMARY OF BACKGROUND DATA

Low back pain is a major physical and socioeconomic entity. A significant percentage of low back pain is attributable to internal disc disruption. The management of internal disc disruption has traditionally been limited to either conservative treatment or spinal fusion. IDET has been performed as an alternative to these therapies.

METHODS

The available literature was reviewed.

RESULTS

Scientific data regarding the pathophysiology, biologic effects, and clinical results are relatively scarce. Early biomechanical and histologic investigations into the effects of IDET are conflicting. However, in early prospective human trials, IDET seems to provide some benefit with little risk.

CONCLUSIONS

IDET is potentially beneficial treatment for internal disc disruption in carefully selected patients as an alternative to spinal fusion. More basic science and clinical research with long-term follow-up evaluation is necessary.

Dual extensor tendon entrapment in Galeazzi fracture-dislocation: a case report

A rare case of blocked reduction of the distal radio-ulnar joint in a Galeazzi fracture-dislocation is presented. In this case both the extensor carpi ulnaris and extensor digiti minimi tendons were displaced on either side of the ulnar head. After relocation of these tendons the distal radio-ulnar joint could be easily reduced.

Biomechanical comparison of lumbar spine with or without spina bifida occulta. A finite element analysis

STUDY DESIGN

Biomechanical study using finite element model (FEM) of lumbar spine.

OBJECTIVES

Very high coincidence of spina bifida occulta (SBO) has been reported more than in 60% of lumbar spondylolysis. The altered biomechanics due to SBO is one considerable factor for this coincidence. Thus, in this study, the biomechanical changes in the lumbar spine due to the presence of SBO were evaluated.

SETTING

United States of America (USA).

METHODS

An experimentally validated three-dimensional nonlinear FEM of the intact ligamentous L3-S1 segment was used and modified to simulate two kinds of SBO at L5. One model had SBO with no change in the length of the spinous process and the other had a small dysplastic spinous process. Von Mises stresses at pars interarticularis were analyzed in the six degrees of lumbar motion with 400 N axial compression, which simulates the standing position. The range of motion at L4/5 and L5/S1 were also calculated.

RESULTS

It was observed that the stresses in all the models were similar, and there was no change in the highest stress value when compared to the intact model. The range of motion was also similar in all the models. The lumbar kinematics of SBO was thus shown to be similar to the intact model.

CONCLUSION

SBO does not alter lumbar biomechanics with respect to stress and range of motion. The high coincidence of spondylolysis in spines with SBO may not be due to the mechanical factors.

Over-distraction of the radio-carpal and mid-carpal joints following external fixation of comminuted distal radial fractures

Seven patients with Frykman type 7 and 8 distal radial fractures, who had been treated with external fixation, were noted to have significant over-distraction of 5-8 mm in the radio-carpal and mid-carpal joints. One patient, who also had abnormal negative ulnar variance of 2 mm, had a fair result using modified Gartland and Werly (1951) criteria. The remaining six patients had good results, indicating that over-distraction does not significantly alter the final outcome, so long as a normal distal radio-ulnar relationship is maintained.

Biomechanical Rationale of Endoscopic Decompression for Lumbar Spondylolysis as an Effective Minimally Invasive Procedure - A Study Based on the Finite Element Analysis

We evaluated the biomechanical behavior of the endoscopic decompression for lumbar spondylolysis using the finite element technique. An experimentally validated, 3-dimensional, non-linear finite element model of the intact L3 - 5 segment was modified to create the L4 bilateral spondylolysis and left-sided endoscopic decompression. The model of Gill's laminectomy (conventional decompression surgery of the spondylolysis) was also created. The stress distributions in the disc and endplate regions were analyzed in response to 400 N compression and 10.6 Nm moment in clinically relevant modes. The results were compared among three models. During the flexion motion, the pressure in the L4/5 nucleus pulposus was 0.09, 0.09 and 0.16 (MPa) for spondylolysis, endoscopic decompression and Gill's procedure, respectively. The corresponding stresses in the annulus fibrosus were 0.65, 0.65 and 1.25 (MPa), respectively. The stress at the adjoining endplates showed an about 2-fold increase in the Gill's procedure compared to the other two models. The stress values for the endoscopic and spondylolysis models were of similar magnitudes. In the other motions, i. e., extension, lateral bending, or axial rotation, the results were similar among all of the models. These results indicate that the Gill's procedure may lead to an increase in intradiscal pressure (IDP) and other biomechanical parameters after the surgery during flexion, whereas the endoscopic decompression did not change the segment mechanics after the surgery, as compared to the spondylolysis alone case. In conclusion, endoscopic decompression of the spondylolysis, as a minimally invasive surgery, does not alert mechanical stability by itself.

Computer evaluation of second tarsometatarsal joint dislocation

Using computer-assisted techniques, this study analyzes the mean contact area of the articular surface of the second tarsometatarsal joint. The articular contact area decreased proportionate to the displacement in both males and females, but it was consistently greater in males than in females for all simulated displacements. The reduction in the contact area was the highest with dorsolateral displacement compared with the lateral and dorsal displacements. Dorsolateral displacement of the second metatarsal of 3 mm led to 38.6% reduction in the contact area, compared with 33.1% and 20.2% reduction with lateral and dorsal displacements, respectively. This study shows that even minor degrees of displacement not apparent on plain radiographs lead to significant decrease in the contact area of the second tarsometatarsal joint. Careful evaluation of second tarsometatarsal injuries with computed tomography is recommended to detect minor degrees of displacement.

Percutaneous computed tomographic stabilization of the pathologic sacroiliac joint

Metastases to the sacroiliac joint region can be a source of significant pain in many patients who are terminally ill. Six patients with metastatic lesions in the sacroiliac region who presented with significant posterior pelvic pain were treated with computed tomography-guided insertion of iliosacral screws. All patients reported excellent pain control in the early postoperative period. Computed tomography-guided insertion of iliosacral screws in an area of relatively preserved bone stock provides good purchase of the screws. It is a safe percutaneous procedure and it helps alleviate pain in patients with sacroiliac metastases.

Biomechanical rationale of ossification of the secondary ossification center on apophyseal bony ring fracture: a biomechanical study

BACKGROUND

Apophyseal ring fracture is one of the important pathologies causing low back pain in children and adolescents. Most of the patients are reported to be in the ossification stage of the ring during growth period rather than early cartilaginous ring stage. There is no previous study clarifying the mechanism of the high prevalence of this disorder in the ossification stage. Thus, in this study, we investigated the effects of ossification of the ring on lumbar spine biomechanics.

METHODS

Two three-dimensional finite element pediatric lumbar models were created and analyzed. One model had ossified apophyseal rings and the other one had cartilaginous apophyseal rings. To simulate standing posture, 341N axial compression was applied. Then, 10Nm moment was applied to the model in the six directions of lumbar motion: flexion, extension, lateral bending and axial rotation. Maximum Von Mises stresses in the apophyseal ring were calculated and compared between the two models.

FINDINGS

The maximum stresses were always higher in the bony ring in all lumbar motion at all lumbar levels compared to the cartilaginous ring. The stresses at L4 caudal apophyseal ring in extension were 2.60 and 0.68 (MPa) for bony and cartilaginous rings respectively. In flexion, stresses were 3.95 and 1.49 (MPa), in lateral bending, stresses were 6.75 and 2.66 (MPa), and in axial rotation, stresses were reported to be 3.15 and 1.72 (MPa). Thus, the bony ring was stressed by at least 2-fold more than the cartilaginous ring.

INTERPRETATION

Apophyseal ring has at least two times more stresses in the ossified stage when compared to the cartilaginous stage resulting in frequent fractures at the interface of bone and cartilage.

Anatomic considerations of the lumbar isthmus

STUDY DESIGN

A morphometric study of lumbar isthmus from L1 to L5 on 30 dried lumbar spines was conducted.

OBJECTIVE

To provide anatomic data about the lumbar isthmus and to quantitatively evaluate structural features of the lumbar isthmus and its relationship to adjacent anatomic structures.

SUMMARY OF BACKGROUND DATA

There are very few anatomic studies about the lumbar isthmus, and no study describes the relationship of the lumbar isthmus to its adjacent structures.

METHODS

Direct measurements using digital calipers and a goniometer were taken from 30 dried lumbar spines. Anatomic evaluation focused on the lumbar isthmus and its related structures, the isthmus pedicle, and superior and inferior facets. Seven linear and four angular parameters of the lumbar isthmus were determined.

RESULTS

The length of the superior edge of the isthmus gradually increased from L2 to L5 (from 8.22 +/- 1.43 mm at L2 to 10.44 +/- 1.90 mm at L5), and that of its inferior edge progressively decreased from L2 to L5 (from 8.67 +/- 1.76 mm at L2 to 6.34 +/- 1.74 mm at L5). The superoinferior diameter of the isthmus decreased from L3 to L5 (from 13.87 +/- 1.77 mm at L3 to 13.26 +/- 2.49 mm at L5). The superior edge of the isthmus was the thinnest at L4 (1.62 +/- 0.58 mm), and its thickness inferiorly increased from L1 to L5 (from 6.71 +/- 1.47 mm at L1 to 7.76 +/- 1.08 mm at L5). The medial and caudal inclination of the isthmus with respect to the pedicle gradually increased from L1 to L5 (from 112.3 degrees +/- 13.8 degrees at L1 to 119.2 degrees +/- 11.2 degrees at L5 medial inclination and from 132.5 degrees +/- 8.8 degrees at L2 to 139.0 degrees +/- 12.1 degrees at L5 caudal inclination, respectively). The dimensions of the lumbar isthmus were positively correlated to dimensions of the pedicle and orientations of the facets.

CONCLUSIONS

This study provides detailed anatomic data of the lumbar isthmus. Anatomic parameters of the lumbar isthmus are related to the vertebral levels and have a significant correlation with the angles of the facets and the dimensions of the pedicles. The vulnerability of the pars interarticularis of the fifth lumbar vertebra has been anatomically confirmed.

Ipsilateral supracondylar fracture of humerus and forearm bones in children

A total of 34 children with ipsilateral supracondylar fractures of the humerus and forearm were studied over an 8-year period. Of these, 19 patients had fractures of the distal quarter of the forearm bones while eight patients had a distal radial epiphyseal injury. Five of the patients had undisplaced supracondylar fractures. One patient had an anterior supracondylar fracture. All forearm fractures were treated by closed reduction. Nine displaced supracondylar fractures which could not be reduced by closed manipulation were treated by olecranon pin traction in two cases and by percutaneous pinning in seven cases. Excellent or good results were found in 29 children after an average follow-up of 3.8 years.

Vulnerability of great medullary artery

STUDY DESIGN

The present study describes anatomic observations on great medullary artery and intercostal arteries pertinent to thoracolumbar spinal surgery.

OBJECTIVES

This study reveals the vulnerable course of the great medullary artery and its relationship to the lateral or posterolateral approach to thoracic spine.

SUMMARY OF BACKGROUND DATA

There are no previous anatomic data on the length of the great medullary artery, its intradural course, its relationship with the anterior spinal artery, and the distance between two adjacent intercostal arteries.

METHODS

The location of the intercostal arteries was defined, and the distance between two adjacent arteries was measured at a point on the lateral surface of the vertebra midway between its anteroposterior diameter. The intradural length of the great medullary artery and the angle it formed with the anterior spinal artery at the point of anastomosis were also measured.

RESULTS

The mean intradural length of the great medullary artery was 3.6 cm (range, 1.7-8.1 cm), and it passed over 1-3 disc spaces before joining the anterior spinal artery at a mean angle of 20.1 degrees (range, 12-28 degrees). The average distance between two adjacent intercostal arteries from T6 to L2 was 3.6 cm (range, 2.8-4.0 cm), which provides a safe window through which a herniated thoracic disc may be approached if surgery is indicated.

CONCLUSIONS

The acute angle between the great medullary artery and anterior spinal artery indicates that these two arteries are in close proximity for considerable length and are liable to be compressed together with the intervening vascular collaterals by a space-occupying lesion, such as disc herniation or a fractured fragment. The longer the intradural course of the great medullary artery, the more vulnerable it is to compression by disc herniation or fracture. The intercostal and lumbar arteries are located at the midportion of the lateral aspect of the vertebral bodies rather than at the level of intervertebral discs. Discectomy or decompression of the anterior thoracic canal may be accomplished through a lateral or posterolateral extracavitary approach between two intercostal or lumbar arteries.

Flexor to extensor tendon transfer for curly toes. 43 children reviewed after 8 (1-25) years

43 children treated by flexor-to-extensor transfer for a total of 130 curly toes were retrospectively reviewed after a mean period of 8 years. According to an objective scoring system, 37 patients had a satisfactory result and 6 patients had a poor result of one or more toes. We now recommend that children with mild to moderate curly toes should be observed until the age of 6 years, by which time the majority will have had spontaneous correction. The remaining patients should be operated on at that stage because children operated on later had poor results. Attention should be paid to the surgical technique because the long flexor tendons are often bipartite.

Anatomic considerations of the principal nutrient foramen and artery on internal surface of the ilium

Direct measurements of the nutrient foramen of thirty dried ilia using digital calipers and observations of the nutrient a. from ten cadaveric specimens were made in the present study. The nutrient foramen was situated 12.5 +/- 2.7 mm lateral to the anterosuperior sacroiliac joint line but perpendicular to this line and 23.5 +/- 5.8 mm above the pelvic brim parallel to the sacroiliac joint line. The nutrient a. originated from the iliolumbar a. as it coursed across the anterosuperior aspect of the sacroiliac joint. The present anatomic study indicates that the nutrient a. on the internal surface of the ilium is prone to injury as a result of traumatic disruption of the sacroiliac joint, sacral alar fractures and during the anterior approach to the sacroiliac joint.

Effects of slip severity and loading directions on the stability of isthmic spondylolisthesis: a finite element model study

STUDY DESIGN

Using a validated finite element model, the biomechanical effects of pars defect in a lumbar segment with and without different degrees of slip (up to 50% slip) were studied.

OBJECTIVES

To study the effects of slip severity and loading parameters on the stability of the lytic and adjacent motion segments. Better knowledge of the biomechanics of spondylolisthesis may help formulate treatment strategies such as bracing or spinal implants.

SUMMARY OF THE BACKGROUND DATA

Clinically, spondylolisthesis exists in varying grades of anterior slip, and the biomechanical stability of the motion segments at the lytic defect and adjacent level probably varies as well. In vitro studies of L4-L5 and L5-S1 isthmic spondylolisthesis slips have concluded that an L4-L5 pars defect is more unstable than an L5-S1 pars defect. Comparing the stability of lytic motion segments with different grades of spondylolisthesis is difficult to do experimentally and therefore has not been done. Further assessing the stresses in the bone and intervertebral discs at or adjacent to a lytic defect is also difficult to study experimentally, so no data are available.

METHODS

A finite element model of L4-S1 was validated with and without a pars defect at L5. The model was then revised to represent different degrees of slip at L5, and six different moment loadings were applied.

RESULTS

The current study showed larger decrease in stiffness with increasing percent slip. The decrease in disc stiffness and increase in disc stresses with increasing percent slip were larger at the level of spondylolisthesis as compared to the changes in the adjacent segment. Lateral bending moment and torsion load showed the largest decrease in stiffness due to slip. At 50% slip, the maximum increase in motion (as compared to motion in an intact segment) was seen under lateral bending moment load (about 55% at L4/L5 and 250% at L5/S1). Lateral bending also produced the largest increase in stresses due to 50% slip in the anulus and endplates (300% increase in anular stress and 190% increase in endplate stress) at L5/S1.

CONCLUSIONS

The stiffness of a spondylolisthetic motion segment decreases as the slip increases. Lateral bending and torsion are moment directions causing the greatest resulting motions.

A pitfall of coronal computed tomographic imaging in evaluation of calcaneal fractures

Computed tomographic (CT) coronal images of the posterior talocalcaneal joint were compared with lateral radiographic views and intraoperative findings in 35 patients with 36 intra-articular calcaneal fractures. The severity of articular incongruity and rotational displacement of the posterior facet fragment were not well appreciated in coronal CT images of nine patients, although Bohler and Gissane angles were significantly decreased in lateral radiographs. This disparity between the radiographic and coronal CT scans can be explained by the intraoperative finding of rotation of the fractured central or lateral portion of the convex posterior calcaneal facet along a horizontal axis in the coronal plane.

Distribution of nerve fibers in the standard incision for carpal tunnel decompression

Twenty-two paired biopsy specimens of skin and subcutaneous tissue from the proximal and distal halves of the conventional curvilinear incision or carpal tunnel decompression were histologically examined. The specimens were immunohistochemically stained with S100 antibody to highlight the nerve fibers. The mean count of free nerve endings in the proximal biopsy site was 4.42/mm2 (SD, 2.97; range, 1.23-12.27), compared to 4.2/mm2 (SD, 2.71; range, 1.01-10.50) in the distal biopsy specimens. This difference was not statistically significant (p = .20, Wilcoxon's signed ranks [matched pairs] test). The proximal incision site for carpal tunnel decompression did not appear to be more neuroreceptive than the distal incision site, providing no support for the implication of proximal incision sites in proximal scar tenderness.

Determination of medial epicondylar epiphyseal angle for supracondylar humeral fractures in children

A new radiographic angle, the medial epicondylar epiphyseal (MEE) angle, was studied in 100 anteroposterior (AP) radiographs of normal children aged 3-12 years. The mean value was 38.2 degrees +/- 4.17 degrees (SD) (range 25 degrees-46 degrees), and the MEE angle could be drawn satisfactorily in 96 radiographs. The MEE angle did not vary significantly from this value in 25 Jones' view radiographs (paired t test, t = 0.06). The MEE angle was useful in assessing the accuracy of reduction of 20 supracondylar fractures of the humerus with satisfactory final results.

Percutaneous pinning for radial neck fractures

Eight patients with angulated radial neck fractures were successfully treated by percutaneous reduction and pinning. The operative technique is simple and is not associated with any important complications. This technique may however not be suitable for radial neck fractures with major translocation.

Ipsilateral Monteggia equivalent injury and distal radial and ulnar fracture in a child

A rare, previously undescribed combination of ipsilateral type III Monteggia equivalent and distal radial and ulnar fracture in a child is reported. A good result was obtained by nonoperative treatment.

Minimally Invasive Decompression for Lumbar Spinal Canal Stenosis in Younger Age Patients Could Lead to Higher Stresses in the Remaining Neural Arch - A Finite Element Investigation

BACKGROUND AND PURPOSE

A young patient group with the symptoms of acquired spinal stenosis has been identified recently in the literature. The patients between 25-50 years of age were found to have signs of lumbar spinal stenosis because of degenerative spinal changes. Some of them were operated on using the same limited decompression approaches as the older patients. However, this group differs from the geriatric population due to the scarcity of remodeling degenerative signs at the spine. Therefore, the possible ligamentous laxity, facet joint degeneration or only the removal of some spinal structures could lead to the increased stresses in the remaining spinal arch and could have an unfavorable course of events after the procedure. A biomechanical study has been done using an experimentally validated finite element model (FEM) of the intact L3-S1 lumbar spine to elucidate the influence of the limited decompression on range of motion (ROM) and stress distribution on the neural arch in this patient group.

METHODS

We simulated unilateral laminotomy L4 and medial facetectomy L4-5, medial facetectomy L4-5 and lateral fenestration of L5 pars interarticularis, combined transarticular lateral and medial approach with partial facetectomy L4-5, "port-hole" decompression at the L4 level, and hemilaminectomy L4 with medial facetectomy L4-5. The ROM and maximum von Mises stresses were analyzed in flexion, extension, lateral bending, and axial rotation in response to a 10.6 Nm moment with 400 N axial compression. The data were compared with the intact spine and hemilaminectomy L4 with medial facetectomy L4-5 models.

RESULTS AND CONCLUSION

The investigation revealed almost the same ROM after simulation but a considerable increase in stresses at both the pars interarticularis and the inferior facet after limited decompressions, especially in extension and rotation to the contralateral side. Stresses at the contralateral L4 pedicle were highest after L4 hemilaminectomy and medial facetectomy L4-5. Due to the observed increases in stresses, the surgeon should be aware of the possibilities of stress-fractures in this patient group.