The role of pediculolaminar fixation in compromised pedicle bone

Management of Osteoporotic Vertebral Fracture: Review Update 2022

A vertebral fracture is the most common type of osteoporotic fracture. Osteoporotic vertebral fractures (OVFs) cause a variety of morbidities and deaths. There are currently few "gold standard treatments" outlined for the management of OVFs in terms of quantity and quality. Conservative treatment is the primary treatment option for OVFs. The treatment of pain includes short-term bed rest, analgesic medication, anti-osteoporotic medications, exercise, and a brace. Numerous reports have been made on studies for vertebral augmentation (VA), including vertebroplasty and kyphoplasty. There is still debate and controversy about the effectiveness of VA in comparison with conservative treatment. Until more robust data are available, current evidence does not support the routine use of VA for OVF. Despite the fact that the majority of OVFs heal without surgery, 15%-35% of patients with an unstable fracture, persistent intractable back pain, or severely collapsed vertebra that causes a neurologic deficit, kyphosis, or chronic pseudarthrosis frequently require surgery. Because no single approach can guarantee the best surgical outcomes, customized surgical techniques are required. Surgeons must stay current on developments in the osteoporotic spine field and be open to new treatment options. Osteoporosis management and prevention are critical to lowering the risk of future OVFs. Clinical studies on bisphosphonate's effects on fracture healing are lacking. Teriparatide was intermittently administered, which dramatically improved spinal fusion and fracture healing while lowering mortality risk. According to the available literature, there are no standard management methods for OVFs. More multimodal approaches, including conservative and surgical treatment, VA, and medications that treat osteoporosis and promote fracture healing, are required to improve the quality of the majority of guidelines.

Current Concepts in the Management of Osteoporotic Vertebral Fractures: A Narrative Review

Vertebral fractures are the most common type of osteoporotic fracture and can increase morbidity and mortality. To date, the guidelines for managing osteoporotic vertebral fractures (OVFs) are limited in quantity and quality, and there is no gold standard treatment for these fractures. Conservative treatment is considered the primary treatment option for OVFs and includes pain relief through shortterm bed rest, analgesics, antiosteoporotic drugs, exercise, and braces. Studies on vertebral augmentation (VA) including vertebroplasty and kyphoplasty have been widely reported, but there is still debate and controversy regarding the effectiveness of VA when compared with conservative treatment, and the routine use of VA for OVF is not supported by current evidence. Although most OVFs heal well, approximately 15%-35% of patients with unstable fractures, chronic intractable back pain, severely collapsed vertebra (leading to neurological deficits and kyphosis), or chronic pseudarthrosis frequently require surgery. Given that there is no single technique for optimizing surgical outcomes in OVFs, tailored surgical techniques are needed. Surgeons need to pay attention to advances in osteoporotic spinal surgery and should be open to novel thoughts and techniques. Prevention and management of osteoporosis is the key element in reducing the risk of subsequent OVFs. Bisphosphonates and teriparatide are mainstay drugs for improving fracture healing in OVF. The effects of bisphosphonates on fracture healing have not been clinically evaluated. The intermittent administration of teriparatide significantly enhanced spinal fusion and fracture healing and reduced mortality risk. Based on the current literature, there is still a lack of standard management strategies for OVF. There is a need for greater efforts through multimodal approaches including conservative treatment, surgery, osteoporosis treatment, and drugs that promote fracture healing to improve the quality of the guidelines.

Balancing rigidity and safety of pedicle screw fixation via a novel expansion mechanism in a severely osteoporotic model

Many successful attempts to increase pullout strength of pedicle screws in osteoporotic bone have been accompanied with an increased risk of catastrophic damage to the patient. To avoid this, a single-armed expansive pedicle screw was designed to increase fixation strength while controlling postfailure damage away from the nerves surrounding the pedicle. The screw was then subsequently tested in two severely osteoporotic models: one representing trabecular bone (with and without the presence of polymethylmethacrylate) and the other representing a combination of trabecular and cortical bone. Maximum pullout strength, stiffness, energy to failure, energy to removal, and size of the resulting block damage were statistically compared among conditions. While expandable pedicle screws produced maximum pullout forces less than or comparable to standard screws, they required a higher amount of energy to be fully removed from both models. Furthermore, damage to the cortical layer in the composite test blocks was smaller in all measured directions for tests involving expandable pedicle screws than those involving standard pedicle screws. This indicates that while initial fixation may not differ in the presence of cortical bone, the expandable pedicle screw offers an increased level of postfailure stability and safety to patients awaiting revision surgery.

Surgical stabilization of the spine in the osteoporotic patient

Osteoporosis affects millions of US citizens, and millions more are at risk for developing the disease. Several operative techniques are available to the spine surgeon to provide care for those affected by osteoporosis. The types of osteoporosis, common surgical complications, medical optimization, and surgical techniques in the osteoporotic spine are reviewed, with an emphasis on preoperative planning.

Design and biomechanical testing of pedicle screw for osteoporotic incidents

In this study, geometrical features of pedicle screws have been modified and their performances are compared. Performance analysis has been made in terms of pull-out strength and torsional strength. The parameters investigated are core diameter, holes drilled normal to screw axis, angle between sequential holes and distance between holes. Three different core diameter have been studied, which are 4 mm (normal core diameter), 5 mm (medium core diameter) and 5.5 mm (high core diameter). Distance between sequential holes has been arranged such that there is either one hole per pitch or one hole per two pitches. Angle between sequential holes is either 90° or 120°. According to the test results, the screw, with medium core diameter (5 mm) containing one hole per two pitches with 90° angle between sequential holes, has exhibited the optimum performance considering torsional strength and pull-out strength requirements. Its torsional strength is slightly higher than and, when Grade 40 polyurethane foam was used as bone simulating material, its pull-out strength is as good as, an undrilled normal core diameter screw, which is already being used in surgical operations. The fatigue performance of this best performed screw has also been found satisfactory according to the related standard. Its pull-out strength is also tested on a calf vertebra and a promising result has been obtained.

Surgical treatment of osteoporotic thoracolumbar compressive fractures with open vertebral cement augmentation of expandable pedicle screw fixation: a biomechanical study and a 2-year follow-up of 20 patients

BACKGROUND

The incidence of screw loosening increases significantly in elderly patients with severe osteoporosis. Open vertebral cement augmentation of expandable pedicle screw fixation may improve fixation strength in the osteoporotic vertebrae.

MATERIALS AND METHODS

Twenty cadaveric vertebrae (L1-L5) were harvested from six osteoporotic lumbar spines. Axial pullout tests were performed to compare the maximum pullout strength (Fmax) of four methods: 1. Conventional pedicle screws (CPS), 2. Expandable pedicle screws (EPS), 3. Cement augmentation of CPS (cemented-CPS), 4. Cement augmentation of EPS (cemented-EPS). Thirty-six consecutive patients with single-vertebral osteoporotic compressive fractures received posterior decompression and spinal fusion with cemented-CPS (16 cases) or cemented-EPS (20 cases). Plain film and/or CT scan were conducted to evaluate the spinal fusion and fixation effectiveness.

RESULTS

The Fmax and energy absorption of cemented-EPS were significantly greater than three control groups. The mean BMD in the severe osteoporosis group was significantly lower than that in the osteoporosis group (t = 2.04, P = 0.036). In the osteoporosis group, cemented-EPS improved the Fmax by 43% and 21% over CPS and cemented-CPS group. In the severe osteoporosis group, cemented-EPS increased the Fmax by 59%, 22%, and 26% over CPS, EPS, and cemented-CPS, respectively. The clinical results showed that all patients suffered from severe osteoporosis. Six months after operation, the JOA and VAS scores in cemented-EPS group improved from 11.4 ± 2.6 and 7.0 ± 1.4 mm to 24.9 ± 1.6 and 2.1 ± 1.3 mm, respectively. No screw loosening occurred in the cemented-EPS group and spinal fusion was achieved. In the cemented-CPS group, four screws loosened (4.2%) according to the radiolucency. Six months after operation, the JOA and VAS scores improved from 13.1 ± 1.9 and 7.6 ± 1.5 mm to 22.8 ± 2.2 and 2.5 ± 1.6 mm, respectively. No cement leaked into the spinal canal in both groups.

CONCLUSIONS

Cemented-EPS could increase fixation strength biomechanically. It could reduce the risks of screw loosening in patients with severe osteoporosis, requiring instrumented arthrodesis.

The effect of screw insertion angle and thread type on the pullout strength of bone screws in normal and osteoporotic cancellous bone models

Screw fixation can be extremely difficult to achieve in osteoporotic (OP) bone because of its low strength. This study determined how pullout strength is affected by placing different bone screws at varying angles in normal and OP bone models. Pullout tests of screws placed axially, and at angles to the pullout axis (ranging from 10° to 40°), were performed in 0.09 g cm(-3), 0.16 g cm(-3) and 0.32 g cm(-3) polyurethane (PU) foam. Two different titanium alloy bone screws were used to test for any effect of thread type (i.e. cancellous or cortical) on the screw pullout strength. The cancellous screw required a significantly higher pullout force than the cortical screw (p<0.05). For both screws, pullout strength significantly increased with increasing PU foam density (p<0.05). For screws placed axially, and sometimes at 10°, the observed mechanism of failure was stripping of the internal screw threads generated within the PU foam by screw insertion. For screws inserted at 10°, 20°, 30° and 40°, the resistance to pullout force was observed to be by compression of the PU foam material above the angled screw; clinically, this suggests that compressed OP bone is stronger than unloaded OP bone.

Junction kinematics between proximal mobile and distal fused lumbar segments: biomechanical analysis of pedicle and hook constructs

BACKGROUND CONTEXT

Biomechanical studies have demonstrated increased motion in motion segments adjacent to instrumentation or arthrodesis. The effects of different configurations of hook and pedicle screw instrumentation on the biomechanical behaviors of adjacent segments have not been well documented.

PURPOSE

To compare the effect of three different fusion constructs on adjacent segment motion proximal to lumbar arthrodesis.

METHODS

Seven human cadaver lumbar spines were tested in the following conditions: 1) intact; 2) L4-L5-simulated circumferential fusion (CF); 3) L4-L5-simulated fusion extended to L3 with pedicle screws; and 4) L4-L5-simulated fusion extended to L3 with sublaminar hooks. Rotation data at L2-L3, L3-L4, and L4-L5 were analyzed using both load limit control (+/-7.5N.m) and displacement limit control (truncated to the greatest common angular motion of the segments for each specimen).

RESULTS

Both the L3-L4 and L2-L3 motion segments above the L4-L5-simulated CF had significantly increased motion in all loading planes compared with the intact spine, but no significant differences were found between L3-L4 and L2-L3 motion. When the L3-L4 segment was stabilized with pedicle screws, its motion was significantly smaller in flexion, lateral bending, and axial rotation than when stabilized with sublaminar hooks. At the same time, L2-L3 motion was significantly larger in flexion, lateral bending, and axial rotation in the pedicle screw model compared with the sublaminar hook construct.

CONCLUSIONS

The use of sublaminar hooks to stabilize the motion segment above a circumferential lumbar fusion reduced motion at the next cephalad segment compared with a similar construct using pedicle screws. The semiconstrained hook enhancement may be considered if a patient is at a risk of adjacent segment disorders.

Preventing distal pullout of posterior spine instrumentation in thoracic hyperkyphosis: a biomechanical analysis

STUDY DESIGN

An in vitro biomechanical study.

OBJECTIVE

Compare the mechanical behavior of 5 different constructs used to terminate dual-rod posterior spinal instrumentation in resisting forward flexion moment.

SUMMARY OF BACKGROUND DATA

Failure of the distal fixation construct can be a significant problem for patients undergoing surgical treatment for thoracic hyperkyphosis. We hypothesize that augmenting distal pedicle screws with infralaminar hooks or sublaminar cables significantly increases the strength and stiffness of these constructs.

METHODS

Thirty-seven thoracolumbar (T12 to L2) calf spines were implanted with 5 configurations of distal constructs: (1) infralaminar hooks, (2) sublaminar cables, (3) pedicle screws, (4) pedicle screws+infralaminar hooks, and (5) pedicle screws+sublaminar cables. Progressive bending moment was applied to each construct until failure. The mode of failure was noted and the construct's stiffness and failure load determined from the load-displacement curves.

RESULTS

Bone density and vertebral dimensions were equivalent among the groups (F=0.1 to 0.9, P>0.05). One-way analysis of covariance (adjusted for differences in density and vertebral dimension) demonstrated that all of the screw-constructs (screw, screw+hook, and screw+cable) exhibited significantly higher stiffness and ultimate failure loads compared with either sublaminar hook or cable alone (P<0.05). The screw+hook constructs (109+/-11 Nm/mm) were significantly stiffer than either screws alone (88+/-17 Nm/mm) or screw+cable (98+/-13 Nm/mm) constructs, P<0.05. Screw+cable construct exhibited significantly higher failure load (1336+/-328 N) compared with screw constructs (1102+/-256 N, P<0.05), whereas not statistically different from the screw+hook construct (1220+/-75 N). The cable and hook constructs failed by laminar fracture, screw construct failed in uniaxial shear (pullout), whereas the screws+(hooks or wires) failed by fracture of caudal vertebral body.

CONCLUSIONS

Posterior dual rod constructs fixed distally using pedicle screws were stiffer and stronger in resisting forward flexion compared with cables or hooks alone. Augmenting these screws with either infralaminar hooks or sublaminar cables provided additional resistance to failure.

Assessment of different screw augmentation techniques and screw designs in osteoporotic spines

This is an experimental study on human cadaver spines. The objective of this study is to compare the pullout forces between three screw augmentation methods and two different screw designs. Surgical interventions of patients with osteoporosis increase following the epidemiological development. Biomechanically the pedicle provides the strongest screw fixation in healthy bone, whereas in osteoporosis all areas of the vertebra are affected by the disease. This explains the high screw failure rates in those patients. Therefore PMMA augmentation of screws is often mandatory. This study involved investigation of the pullout forces of augmented transpedicular screws in five human lumbar spines (L1-L4). Each spine was treated with four different methods: non-augmented unperforated (solid) screw, perforated screw with vertebroplasty augmentation, solid screw with vertebroplasty augmentation and solid screw with balloon kyphoplasty augmentation. Screws were augmented with Polymethylmethacrylate (PMMA). The pullout forces were measured for each treatment with an Instron testing device. The bone mineral density was measured for each vertebra with Micro-CT. The statistical analysis was performed with a two-sided independent student t test. Forty screws (10 per group and level) were inserted. The vertebroplasty-augmented screws showed a significant higher pullout force (mean 918.5 N, P = 0.001) than control (mean 51 N), the balloon kyphoplasty group did not improve the pullout force significantly (mean 781 N, P > 0.05). However, leakage occurred in some cases treated with perforated screws. All spines showed osteoporosis on Micro-CT. Vertebroplasty-augmented screws, augmentation of perforated screws and balloon kyphoplasty augmented screws show higher pullout resistance than non-augmented screws. Significant higher pullout forces were only reached in the vertebroplasty augmented vertebra. The perforated screw design led to epidural leakage due to the position of the perforation in the screw. The position of the most proximal perforation is critical, depending on screw design and proper insertion depth. Nevertheless, using a properly designed perforated screw will facilitate augmentation and instrumentation in osteoporotic spines.

ADULT DEGENERATIVE SCOLIOSIS

OBJECTIVE

To review and define principles and features of treatment for adult degenerative scoliosis, the most common cause of adult spinal deformities.

STUDY DESIGN

We conducted a comprehensive review of the literature and our clinical experience.

METHODS

A systematic review of Medline was conducted, including journal articles published in March 2007 and before. We searched for articles related to adult spinal deformities (scoliosis) and treatments.

CONCLUSION

Degenerative scoliosis is a complex disorder. The primary surgical aims are to decompress the neural elements, normalize both sagittal balance and coronal and rotational deformity, fixate to the sacrum/ilium when appropriate, and optimize conditions for osteogenesis and fusion.

Biomechanical characteristics of hybrid hook-screw constructs in short-segment thoracic fixation

STUDY DESIGN

Ex vivo biomechanical testing of human cadaveric thoracic spine segments.

OBJECTIVE

To determine whether a hybrid construct, using a combination of pedicle screws (PSs) and lamina hooks, was equivalent to a PS construct, in a short-segment thoracic spine fixation model.

SUMMARY OF BACKGROUND DATA

Comparisons have been made among PS, lamina hook, and hybrid screw-hook constructs, but these have generally been in long-segment scoliosis correction. In this study, we compared the hybrid and screw-only constructs in a short-segment thoracic fixation.

METHODS

For pullout testing, matched specimens were used for PS (n = 8) and hybrid (n = 8) constructs. Construct stiffness, and the force required for construct failure, were measured. Dynamic testing was carried out on specimens in the PS (n = 7) and hybrid (n = 7) groups in compression, flexion, extension, and left and right lateral bending. Each group was tested intact, after instrumentation, and after corpectomy.

RESULTS

When compared with the hybrid group, a significantly greater force was required for construct failure in the PS group, and these PS constructs were significantly stiffer. No differences were found between groups in dynamic testing.

CONCLUSION

A construct employing PSs is significantly stiffer and more resistant to pullout failure than a hook-screw hybrid construct.

Instrumentation-related complications of multilevel fusions for adult spinal deformity patients over age 65: surgical considerations and treatment options in patients with poor bone quality

STUDY DESIGN

Retrospective follow-up of patients over the age of 65 with a minimum of five-level fusions.

OBJECTIVE

To determine the effect on outcomes of long constructs in patients with poor bone stock, and to review surgical techniques used in patients with poor bone stock.

SUMMARY OF BACKGROUND DATA

Scoliotic deformities in patients with poor bone stock require alterations in both the surgical technique and preoperative planning. To our knowledge, complications of long constructs in poor bone stock have not been specifically reported.

METHOD

Patients over the age of 65 that underwent a minimum of five-level fusion over a 5-year period were reviewed. We reviewed both operative reports and clinic notes and recorded both early and late complications.

RESULTS

Early complications included pedicle fractures and compression fractures with an overall rate of 13%. Late complications included pseudarthroses with instrumentation failure, adjacent level disc degeneration with herniation, compression fractures, and progressive kyphosis. Progressive junctional kyphosis occurred in 26% of patients.

CONCLUSIONS

Spinal stabilization surgery in patients with poor bone stock is associated with high complication rates. Complications such as progressive kyphosis adjacent to the fusion are difficult to address with instrumentation alone.

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.

Posterior instrumentation for thoracolumbar fractures

Thoracolumbar fractures are relatively common injuries. Numerous classification systems have been developed to characterize these fractures and their prognostic and therapeutic implications. Recent emphasis on short, rigid fixation has influenced surgical management. Most compression and stable burst fractures should be treated nonsurgically. Neurologically intact patients with unstable burst fractures that have >25 degrees of kyphosis, >50% loss of vertebral height, or >40% canal compromise often can be treated with short, rigid posterior fusions. Patients with unstable burst fractures and neurologic deficits require direct or indirect decompression. Posterior stabilization can be effective with Chance fractures and flexion-distraction injuries that have marked kyphosis, and in translational or shear injuries. Advances in understanding both biomechanics and types of fixation have influenced the development of reliable systems that can effectively stabilize these fractures and permit early mobilization.

Pedicle screw motion in the osteoporotic spine after augmentation with laminar hooks, sublaminar wires, or calcium phosphate cement: a comparative analysis

STUDY DESIGN

A biomechanical study addressing the motion of pedicle screws in a human cadaveric, osteoporotic spine model.

OBJECTIVES

To compare the fixation of pedicle screws in an osteoporotic spine model after augmentation with laminar hooks, sublaminar wires, or calcium phosphate cement and to determine the kinematic patterns of these screws.

SUMMARY OF BACKGROUND DATA

Numerous techniques exist for improving the quality of fixation within the osteoporotic spine, including supplementing the construct with laminar hooks, sublaminar wires, or calcium phosphate cement. Direct comparisons of these practices, however, are lacking. METHODS.: Twenty-four cadaveric lumbar vertebrae were instrumented with a pedicle screw and rod construct augmented with laminar hooks, sublaminar wires, or calcium phosphate cement. The screws were tested cyclically with physiologic loads. Rigid body motions of the screws were measured using an optoelectronic camera system, and the motion at the screw tip and at the screw head were calculated. Screw motions were compared using nonparametric paired statistical analysis.

RESULTS

Between augmentation groups, there were no significant differences in the magnitude of motion at the screw head and at the screw tip. After calcium phosphate cement supplementation, screw motion was predominantly rotational in nature, whereas rigid body translation of the screw was more common with sublaminar wires or laminar hooks.

CONCLUSIONS

The three augmentation techniques were similar in their ability to enhance the rigidity of fixation of the pedicle screws. Differences did exist, however, in the patterns of pedicle screw motion, with the calcium phosphate cement augmentation resulting in less rigid body translation than the other two techniques.

Pedicular fixation in the osteoporotic spine: a pilot in vivo study on long-term ovariectomized sheep

Spinal instrumentation success is greatly affected by the presence of osteoporosis. To date, however, no data exist on in vivo investigations on biomaterial and surgical techniques in the osteoporotic spine. In the present study 24 uncoated and 24 HA-coated screws were implanted in the L3, L4 and L5 pedicles of eight sheep (four ovariectomized, OVX Group; four sham-operated, Control Group). At four months, uncoated screws showed a significant decrease of about -22% in the extraction torque of the OVX Group as compared to the Control Group (p < 0.005). The extraction torque of HA-coated screws significantly (p < 0.0005) improved in both groups when compared to that of uncoated screws and showed increases ranging from 133% to 157%. Pedicle trabecular bone of OVX sheep showed a significant decrease in BV/TV (-30%; p < 0.05) and Tb.Th (-33%; p < 0.01). The affinity index (AI) results revealed significant (p < 0.0005) differences between uncoated and HA-coated screws for both groups: values were lower for uncoated than HA-coated screws by about -35%. A significant difference was also found for the AI data of uncoated screws between the OVX and Control Groups (-13%, p < 0.005). The current findings have demonstrated that long-term ovariectomized sheep can be used to study in vivo osteointegration in the osteoporotic spine. The HA coating has proven to improve bone purchase and bone-screw interface strength in healthy and osteopenic animals.

Evaluation of calcium sulfate paste for augmentation of lumbar pedicle screw pullout strength

BACKGROUND CONTEXT

Many authors have evaluated the components responsible for ultimate pullout strength of pedicle screws. In these studies, one important variable has been the screw fixation. Because pedicle screw fixation has increased in popularity over recent years, so has the need for augmentation in difficult situations. Polymethylmethacrylate (PMMA) has been established as the gold standard in terms of strength of fixation but has the potential for severe complications when applied in spine surgery. Calcium sulfate is an alternative to PMMA, because it lacks the exothermic reaction, is potentially osteoconductive and is resorbed by the body in 30 to 60 days.

PURPOSE

To determine the strength of a new calcium sulfate cement in terms of pedicle screw augmentation. The purpose was to evaluate calcium sulfate versus PMMA in terms of pullout strength. PMMA was considered the gold standard in terms of strength for this experiment.

STUDY DESIGN

Lumbar vertebrae implanted with pedicle screws were subjected to axial pullout tests. The force required to cause implant failure was measured and compared for three methods of fixation.

OUTCOME MEASURES

Force to failure (Newtons) for each pedicle test was recorded and analyzed with results from similarly augmented pedicles.

METHODS

Lumbar vertebrae were harvested from four cadavers and implanted with pedicle screws. These screws were either placed in native bone or augmented with either calcium sulfate paste or PMMA. In those pedicles that had augmentation, the material was permitted to set for a minimum of 24 hours. Axial pullout tests were then performed using an MTS (Materials Testing System Corporation, Minneapolis, MN) testing machine. The screws were pulled out over a distance of up to 6 mm at 0.25 mm/sec. This rate and distance ensured failure in each case. The load to failure was recorded for each pedicle.

RESULTS

Calcium sulfate augmentation improved pedicle screw pullout strength significantly when compared with native bone (p=.0003). This represented an average increase of 167% over the native bone. Likewise, PMMA improved the pullout strength significantly over native bone (p<.0001) for an average increase of 199%. There were no significant differences between the calcium sulfate and PMMA augmentation (p=.0791), although the PMMA averaged 119% of the strength of calcium sulfate.

CONCLUSIONS

Although PMMA is considered the gold standard in terms of fixation strength, its use around the spine has been limited because of concern over complications. The calcium sulfate bone paste tested in this study demonstrated strength similar to PMMA without the major risks to the spinal cord. In addition, the calcium sulfate paste allows for possible osteoconduction to aid in spinal fusion. Further study is needed to determine the applicability of this calcium sulfate paste in the clinical setting of spine surgery.

Influence of bone mineral density on pedicle screw fixation: a study of pedicle screw fixation augmenting posterior lumbar interbody fusion in elderly patients

BACKGROUND CONTEXT

Some biomechanical studies have demonstrated that bone mineral density of the lumbar spine (BMD) affects the stability of pedicle screws in vitro.

PURPOSE

To investigate influence of BMD on loosening and related failure of pedicle screws in vivo.

STUDY DESIGN/SETTING

A clinical study of 52 patients who underwent pedicle screw fixation augmenting posterior lumbar interbody fusion (PLIF).

PATIENT SAMPLE

There were 13 men and 39 women, with an average age of 63 years (range, 45-76 years) at the time of operation. The mean follow-up period was 2.8 years (range, 2-6 years).

OUTCOME MEASURES

Relationship between BMD, screw loosening, and its related failures were statistically analyzed.

METHODS

BMD was measured by the dual energy X-ray absorptiometry (DEXA) method. Radiographic assessments were done by the first author and independently by another orthopedist who was not informed of the values of BMD.

RESULTS

The mean BMD of all patients was 0.879 +/- 0.215 (mean +/- S.D.) g/cm2. The mean BMD in patients with and without screw loosening was 0.720 +/- 0.078 g/cm2 (n=11) and 0.922 +/- 0.221 g/cm2 (n=41). There was a significant difference between the mean BMD of patients with and without screw loosening (P<.01). The mean BMD of patients with "union," "nonunion" and "undetermined union" was 0.934 +/- 0.210 g/cm2 (n=40), 0.674 +/- 0.104 g/cm2 (n=4) and 0.710 +/- 0.116 g/cm2 (n=8), respectively. The mean BMD of patients with "union" was significantly greater than those with "nonunion" and "undetermined union" (P<.05).

CONCLUSION

It could be concluded that BMD has a close relation with the stability of pedicle screws in vivo, and BMD value below 0.674 +/- 0.104 g/cm2 suggests a potential increased risk of "nonunion" when pedicle screw fixation is performed in conjunction with PLIF.

Can insertional torque predict screw loosening and related failures? An in vivo study of pedicle screw fixation augmenting posterior lumbar interbody fusion

STUDY DESIGN

An investigation of the relation between intraoperative insertional torque of pedicle screws, bone mineral density of the vertebra, and development of screw loosening in vivo.

OBJECTIVES

To determine the usefulness of intraoperative measurement of the insertional torque of pedicle screws.

SUMMARY OF BACKGROUND DATA

Some biomechanical studies have demonstrated that the insertional torque is highly correlated with bone mineral density and the stability of pedicle screws in vitro.

METHODS

Pedicle screw fixation was performed with posterior lumbar interbody fusion in 62 consecutive patients. The mean age of the patients at the time of surgery was 58 years. The insertional torque of pedicle screws was measured intraoperatively in all patients. The mean follow-up period was 2.7 years.

RESULTS

The mean insertional torque was 1.28 +/- 0.37 Nm in patients with screw loosening and 1.50 +/- 0. 40 Nm in patients without the problem. The mean insertional torque in patients with compression fractures in the upper vertebra adjacent to the fixed segment was 0.83 +/- 0.23 Nm. There was no significant difference between the mean insertional torque in patients with screw loosening and those without the condition. The mean insertional torque in patients without screw loosening was significantly greater than in patients with compression fractures (P < 0.01). A high correlation was found between insertional torque and bone mineral density (P < 0.01).

CONCLUSIONS

Although a high correlation was found between the insertional torque of pedicle screws and bone mineral density in vivo, the insertional torque could not objectively predict screw loosening.

Restoration of pedicle screw fixation with an in situ setting calcium phosphate cement

STUDY DESIGN

Pedicle screws were pulled out of human cadaveric vertebrae before and after augmentation with polymethylmethacrylate or in situ-setting calcium phosphate cement. The fixation strength of screws augmented with calcium phosphate cement was compared with that of screws augmented with polymethylmethacrylate.

OBJECTIVES

To determine whether a new in situ-setting calcium phosphate cement might be suitable for augmenting the fixation of pedicle screws. The principle objective was to compare the pull-out resistance of screws augmented with calcium phosphate cement with the pull-out behavior of screws augmented with polymethylmethacrylate. Polymethylmethacrylate augmentation was chosen as the standard because of its current clinical use. Five types of screws were tested to determine whether screw design had an effect on the efficacy of augmentation.

SUMMARY OF BACKGROUND DATA

Although many factors affect the pull-out resistance of pedicle screws, a key determinant of their performance is the strength of their attachment to the spine. In elderly, osteopenic patients, the screw-bone interface is especially at risk for stripping during insertion or pull-out after surgery. In these patients, polymethylmethacrylate has been used to augment pedicle screw fixation, although its use is not without risk. In situ-setting calcium phosphate cements may provide an alternative to polymethylmethacrylate in this application. Like polymethylmethacrylate, calcium phosphate cements can be injected into the prepared screw hole. They have the added advantage of being resorbed and replaced during healing and normal bone remodeling.

METHODS

Thirty human lower lumbar vertebrae (L3-L5) were implanted bilaterally with one of five types of pedicle screws (n = 6 for each screw type). The screws were pulled out 3.0 mm at 0.25 mm/sec with a servohydraulic materials testing machine. The 3.0-mm pull-out distance, which was slightly longer than one thread pitch, was designed to strip the screw-bone interface but to leave the pedicle otherwise intact. After the initial testing, the screws in each vertebrae were removed, and the screw tracks were filled with 2.0 cc of polymethylmethacrylate (one side) or calcium phosphate cement (contralateral side). After augmentation, the screws were reinserted, and the cements were allowed to harden for 24 hours. Postaugmentation testing followed the protocols for preaugmentation testing, and the pull-out resistance of screws augmented with calcium phosphate cement was compared with the pull-out resistance of screws augmented with polymethylmethacrylate.

RESULTS

Mechanically, calcium phosphate cement compared favorably with polymethylmethacrylate for augmenting pedicle screws. Both restored the strength of the screw-bone interface: across all screw types, the average increase in pull-out strength was 147% with polymethylmethacrylate augmentation and 102% with calcium phosphate cement. There were no significant differences because of screw type with either type of augmentation.

CONCLUSIONS

The in situ-setting calcium phosphate cement investigated in this study compared favorably with polymethylmethacrylate in a single-cycle, pull-out test of augmented pedicle screws in senile trabecular bone. With further evaluation, this cement may offer an alternative to polymethylmethacrylate for the enhancement of pedicle screw fixation clinically.