The mechanical role of laminar hook protection of pedicle screws at the caudal end vertebra

Surgical Management of Spinal Conditions in the Elderly Osteoporotic Spine

Osteoporosis, the most common form of metabolic bone disease, leads to alterations in bone structure and density that have been shown to compromise the strength of spinal instrumentation. In addition, osteoporosis may contribute to high rates of fracture and instrumentation failure after long posterior spinal fusions, resulting in proximal junctional kyphosis and recurrent spinal deformity. As increasing numbers of elderly patients present for surgical intervention for degenerative and traumatic spinal pathologies, current and future generations of spine surgeons will increasingly be faced with the challenge of obtaining adequate fixation in osteoporotic bone. The purpose of this review is to familiarize the reader with the impact of osteoporosis on spinal instrumentation, the broad variety of techniques that have been developed for addressing these issues, and the biomechanical and clinical evidence in support of the use of these techniques.

Biomechanical comparison of different combinations of hook and screw in one spine motion unit--an experiment in porcine model

BACKGROUND

The biomechanical performance of the hooks and screws in spinal posterior instrumentation is not well-characterized. Screw-bone interface failure at the uppermost and lowermost vertebrae is not uncommon. Some have advocated for the use of supplement hooks to prevent screw loosening. However, studies describing methods for combined hook and screw systems that fully address the benefits of these systems are lacking. Thus, the choice of which implant to use in a given case is often based solely on a surgeon's experience instead of on the biomechanical features and advantages of each device.

METHODS

We conducted a biomechanical comparison of devices instrumented with different combinations of hooks and screws. Thirty-six fresh low thoracic porcine spines were assigned to three groups (12 per group) according to the configuration used for of fixation: (1) pedicle screw; (2) lamina hook and (3) combination of pedicle screw and lamina hook. Axial pullout tests backward on transverse plane in the direction normal to the rods were performed using a material testing machine and a specially designed grip with self-aligned function.

RESULTS

The pullout force for the pedicle screws group was significantly greater than for the hooks and the combination (p < 0.05). However, no significant difference was found between the hooks and the combination (p > 0.05).

CONCLUSIONS

Pedicle screws achieve the maximal pullout strength for spinal posterior instrumentation.

Biomechanical comparison of different combinations of hook and screw in one spine motion unit--an experiment in porcine model

Background

The biomechanical performance of the hooks and screws in spinal posterior instrumentation is not well-characterized. Screw-bone interface failure at the uppermost and lowermost vertebrae is not uncommon. Some have advocated for the use of supplement hooks to prevent screw loosening. However, studies describing methods for combined hook and screw systems that fully address the benefits of these systems are lacking. Thus, the choice of which implant to use in a given case is often based solely on a surgeon’s experience instead of on the biomechanical features and advantages of each device.

Methods

We conducted a biomechanical comparison of devices instrumented with different combinations of hooks and screws. Thirty-six fresh low thoracic porcine spines were assigned to three groups (12 per group) according to the configuration used for of fixation: (1) pedicle screw; (2) lamina hook and (3) combination of pedicle screw and lamina hook. Axial pullout tests backward on transverse plane in the direction normal to the rods were performed using a material testing machine and a specially designed grip with self-aligned function.

Results

The pullout force for the pedicle screws group was significantly greater than for the hooks and the combination (p < 0.05). However, no significant difference was found between the hooks and the combination (p > 0.05).

Conclusions

Pedicle screws achieve the maximal pullout strength for spinal posterior instrumentation.

A biomechanical assessment of infra-laminar hooks as an alternative to supra-laminar hooks in thoracolumbar fixation

STUDY DESIGN

Biomechanical study, using human cadaveric spines, of two types of posterior fixation in the thoracolumbar spine.

OBJECTIVES

To compare fixation in the thoracolumbar spine using pedicle screws combined with infra-laminar hooks (at both the cranial and caudal ends of the same vertebra) with fixation using pedicle screws alone.

SUMMARY OF BACKGROUND DATA

The standard method of using laminar hooks is to place supra-laminar hooks at the level above the vertebra being fixed by the cranial pedicle screws, and infra-laminar hooks at the level of the vertebra receiving the caudal pedicle screws. However, using this method, a single normal motion segment is killed. In the proposed technique, infra-laminar hooks will be used at the same level as the cranial pedicle screws, thus sparing the superior adjacent segment.

METHODS

PEDICLE SCREW LOOSENING TEST: Nine vertebrae (T9-L2) had pedicle screws inserted into both pedicles. On one side, the pedicle screw was connected to a fixation rod, while on the other side the pedicle screw was combined with an infra-laminar hook and connected to a rod. The rod on one side was then pushed with increasing force until loosening of the screw. The same test was repeated on the other rod until loosening of the screw. The reverse test (i.e., pulling until loosening) was done in a similar manner on another nine vertebrae. BIOMECHANICAL STIFFNESS TEST: Eight spines (T12-L2) were biomechanically tested in: axial compression, flexion, extension, left and right lateral bending, and left and right axial torsion. This sequence was applied to: 1) the intact spine; 2) after applying pedicle screws and infra-laminar hooks with rods to the destabilized spine; and 3) after removal of the hooks (with the pedicle screw and rods still inserted). From the load-deformation curves obtained, stiffness values were calculated.

RESULTS

PEDICLE SCREW LOOSENING TEST: The pedicle screw loosening tests showed that a pedicle screw combined with an infra-laminar hook offers significantly greater fixation strength as compared to a pedicle screw alone. Biomechanical stiffness test. Fixation by pedicle screws combined with infra-laminar hooks, placed on the vertebra fixed by pedicle screws at both the cranial and caudal ends, offered a stiffer construct as compared to fixation by pedicle screws alone.

CONCLUSIONS

This method offers a firm construct without sacrificing a normal motion segment.

In vitro biomechanical comparison of pedicle screws, sublaminar hooks, and sublaminar cables

OBJECT

Three types of posterior thoracolumbar implants are in use today: pedicle screws, sublaminar titaniumcables, and sublaminar hooks. The authors conducted a biomechanical comparison of these three implants in human cadaveric spines.

METHODS

Spine specimens (T5-12) were harvested, radiographically assessed for fractures or metastases, and their bone mineral density (BMD) was measured. Individual vertebrae were disarticulated and fitted with either pedicle screws, sublaminar cables, or bilateral claw hooks. The longitudinal component of each construct consisted of bilateral 10-cm rods connected with two cross-connectors. The vertebral body was embedded in cement, and the rods were affixed to a ball-and-socket apparatus for the application of a distraction force. The authors analyzed 1) 20 vertebrae implanted with screws; 2) 20 with hooks, and 3) 20 with cables. The maximum pullout (MPO) forces prior to failure (mean +/- standard deviation) for the screw, hook, and cable implants were 972 +/- 330, 802 +/- 356, and 654 +/- 248 N, respectively (p = 0.0375). Cables allowed significantly greater displacement (6.80 +/- 3.95 mm) prior to reaching the MPO force than hooks (3.73 +/- 1.42 mm) and screws (4.42 +/- 2.15 mm [p = 0.0108]). Eleven screw-implanted vertebrae failed because of screw pullout. All hook-and-cable-implanted vertebrae failed because of pedicle, middle column, or laminar fracture.

CONCLUSIONS

These findings suggest that screws possess the greatest pullout strength of the three fixation systems. Sublaminar cables are the least rigid of the three. When screw failure occurred, the mechanism was generally screw back-out, without vertebral fractures.

The use of pedicle-screw internal fixation for the operative treatment of spinal disorders

Pedicle screws have dramatically improved the outcomes of spinal reconstruction requiring spinal fusion. Short-segment surgical treatments based on the use of pedicle screws for the treatment of neoplastic, developmental, congenital, traumatic, and degenerative conditions have been proved to be practical, safe, and effective. The Funnel Technique provides a straightforward, direct, and inexpensive way to very safely apply pedicle screws in the cervical, thoracic, or lumbar spine. Carefully applied pedicle-screw fixation does not produce severe or frequent complications. Pedicle-screw fixation can be effectively and safely used wherever a vertebral pedicle can accommodate a pedicle screw--that is, in the cervical, thoracic, or lumbar spine. Training in pedicle-screw application should be standard in orthopaedic training programs since pedicle-screw fixation represents the so-called gold standard of spinal internal fixation.