In vitro comparison of the pullout strength of 3 anterior double-screw fixation techniques with different screw angulations.

OBJECT: The pullout resistance of double-screw fixation systems in anterior spine surgery has been shown to be dependent on screw length as well as on screw angulation. The objective of the study was to evaluate the pullout strength of anterior double-screw systems with different angulations. METHODS: The authors conducted a comparative pullout test of converging, parallel, and diverging angulations of double-screw systems in human cadavers. Twenty-four human vertebral bodies from T-11 to L-1 were harvested from 8 donors, dissected from surrounding tissue, and matched to 3 different fixation groups. Three systems were tested: VentroFix, with near parallel screw direction; the Hopf Anterior Fixation System (HAFS), with converging screw angulation; and the ART anterior system, with diverging screw angulation. RESULTS: The mean (± SD) pullout strength of the VentroFix system was 699 ± 214 N, whereas the HAFS resisted to 591 ± 372 N. The ART anterior system with diverging screws demonstrated a pullout resistance of 810 ± 273 N. There was no significant difference amongst the pullout forces of the 3 groups (p > 0.05). In the HAFS and the ART anterior group, a weak correlation of pullout strength and bone mineral density measured by quantitative CT was found (r = 0.59 and r = 0.62, respectively), whereas the pullout force of the VentroFix system was not correlated with bone mineral density (r = 0.33). CONCLUSIONS: The in vitro pullout resistance of anterior double-screw systems does not appear to depend on screw angulation.

Vertebral body stenting: a new method for vertebral augmentation versus kyphoplasty.

Vertebroplasty and kyphoplasty are well-established minimally invasive treatment options for compression fractures of osteoporotic vertebral bodies. Possible procedural disadvantages, however, include incomplete fracture reduction or a significant loss of reduction after balloon tamp deflation, prior to cement injection. A new procedure called "vertebral body stenting" (VBS) was tested in vitro and compared to kyphoplasty. VBS uses a specially designed catheter-mounted stent which can be implanted and expanded inside the vertebral body. As much as 24 fresh frozen human cadaveric vertebral bodies (T11-L5) were utilized. After creating typical compression fractures, the vertebral bodies were reduced by kyphoplasty (n = 12) or by VBS (n = 12) and then stabilized with PMMA bone cement. Each step of the procedure was performed under fluoroscopic control and analysed quantitatively. Finally, static and dynamic biomechanical tests were performed. A complete initial reduction of the fractured vertebral body height was achieved by both systems. There was a significant loss of reduction after balloon deflation in kyphoplasty compared to VBS, and a significant total height gain by VBS (mean +/- SD in %, p < 0.05, demonstrated by: anterior height loss after deflation in relation to preoperative height [kyphoplasty: 11.7 +/- 6.2; VBS: 3.7 +/- 3.8], and total anterior height gain [kyphoplasty: 8.0 +/- 9.4; VBS: 13.3 +/- 7.6]). Biomechanical tests showed no significant stiffness and failure load differences between systems. VBS is an innovative technique which allows for the possibly complete reduction of vertebral compression fractures and helps maintain the restored height by means of a stent. The height loss after balloon deflation is significantly decreased by using VBS compared to kyphoplasty, thus offering a new promising option for vertebral augmentation.

Spondylolisthesis caused by extreme pedicle elongation in osteogenesis imperfecta.

Although osteogenesis imperfecta is a well-known skeletal disorder, reports of spondylolisthesis in osteogenesis imperfecta are rare. Only very few cases of spondylolisthesis caused by elongation of lumbar pedicles have been described in the literature. Here we report three patients suffering from osteogenesis imperfecta showing a severe form of hyperlordosis caused by lumbar pedicle elongation and consecutive spondylolisthesis. Radiographs in the course of childhood and adolescence show a rapid progression of pedicle elongation and hyperlordosis with increased mechanical loads. The treatment strategy consists of physiotherapy, medical treatment with bisphosphonates, and orthopedic surgery and is preferably conservative. In the three patients reported here, one patient was treated with laminectomy and postero-lateral fusion, whereas in the other two patients surgery is currently not considered as necessary.

Cementless total hip replacement with subtrochanteric femoral shortening for severe developmental dysplasia of the hip.

INTRODUCTION: Total joint replacement in patients suffering from developmental dysplasia of the hip poses specific technical difficulties due to insufficient bone stock at the site of the original and secondary acetabulum and a narrow, cranially displaced proximal femur. MATERIALS AND METHODS: Twelve hips with severe congenital dislocation (4 Crowe type II, 5 type III, 3 type IV) were treated with cementless, porous structured total hip replacement. The cup was implanted at the anatomic height, a femoral segment was resected below the minor trochanter to reduce the femoral component in all cases. RESULTS: One femoral component was exchanged for a longer stem after 2 months due to insufficient fit and fill. After a mean follow-up of 5.1 years, there were no further revisions or radiographic signs of loosening. No cup was revised or loose radiographically. Harris hip score (mean) improved from 36 to 82 points, mean leg length discrepancy was reduced from 5.4 to 1.3 cm. CONCLUSION: The cementless technique described is useful in cases of severe congenital dislocation of the hip when the cup is implanted at the anatomic level, and implant reduction is not possible despite thorough soft-tissue release or not advisable because of excessive limb lengthening. A reconstruction of the original joint center and good mid-term results are demonstrated.