The effects of screw orientation in severely osteoporotic bone: a comparison with locked plating

Screw Insertional Torque Measurement in Spine Surgery: Correlation With Bone Mineral Density and Hounsfield Unit

OBJECTIVE

Achieving successful fusion during spine surgery is dependent on rigid pedicle screw fixation. To assess fixation strength, the insertional torque can be measured during intraoperative screw fixation. This study aimed to explore the technical feasibility of measuring the insertional torque of a pedicle screw, while investigating its relationship with bone density.

METHODS

Thoraco-lumbar screw fixation fusion surgery was performed on 53 patients (mean age, 65.5 ± 9.8 years). The insertional torque of 284 screws was measured at the point passing through the pedicle using a calibrated torque wrench, with a specially designed connector to the spine screw system. The Hounsfield units (HU) value was determined by assessing the trabecular portion of the index vertebral body on sagittal computed tomography images. We analyzed the relationship between the measured insertional torque and the following bone strength parameters: bone mineral density (BMD) and HU of the vertebral body.

RESULTS

The mean insertion torque was 105.55 ± 58.08 N∙cm and T-score value (BMD) was -1.14 ± 1.49. Mean HU value was 136.37 ± 57.59. Screw insertion torque was positively correlated with BMD and HU in whole patients. However, in cases of osteopenia, all variables showed very weak correlations with insertional torque. In patients with osteoporosis, there was no statistically significant correlation between BMD and torque strength; HU showed a significant correlation.

CONCLUSION

The insertional torque of screw fixation significantly correlated with bone density (BMD and HU). HU measurements showed greater clinical significance than did BMD values in patients with osteoporosis.

Outcomes of Locking-Plate Fixation for Hindfoot Fusion Procedures in 15 Patients

Tibiotalocalcaneal arthrodesis is a salvage procedure for various end-stage foot and ankle pathologic entities. Several factors are known to influence the union rate after these procedures, including construct rigidity. The data on locked plates as a fixation technique have been inconclusive, with variable union rates reported. One recent study suggested that locking plates can lead to high nonunion rates owing to excessive rigidity. The purpose of the present study was to retrospectively examine the outcomes of locking plate fixation. We retrospectively reviewed the cases of 15 patients (7 [46.7%] male, 8 [53.3%] female) who underwent tibiotalocalcaneal, tibiocalcaneal, or tibiotalar arthrodesis fixed with a locking plate from January 2013 to January 2014. The average age was 52.19 ± 5.8 years. The mean follow-up period was 17 ± 5.3 months. We examined the overall union rates and the effects of smoking, diabetes, and rheumatologic status on the union rate. Of the 15 cases, 11 (73.3%) did not achieve union. The mean time to failure was 10 ± 5.3 months. Age, gender, smoking, diabetes, use of augmentation screws outside the plate, and operating surgeon did not have an effect on the failure rate (p > .50). In addition, gender, smoking, and diabetes did not predict for nonunion. The high failure rate of rigid locking plate fixation reported might be attributable to the high incidence of smoking and diabetic comorbidities in our study. However, excessive construct rigidity might play an important role. Larger studies are needed to establish more reliable union rates with the use of locking plates in foot and ankle fusion.

Pre-operative planning for fracture fixation using locking plates: device configuration and other considerations

Most locked plating failures are due to inappropriate device configuration for the fracture pattern. Several studies cite screw positioning variables such as the number and spacing of screws as responsible for occurrences of locking plate breakage, screw loosening, and peri-prosthetic re-fracture. It is also widely accepted that inappropriate device stiffness can inhibit or delay healing. Careful preoperative planning is therefore critical if these failures are to be prevented. This study examines several variables which need to be considered when optimising a locking plate fixation device for fracture treatment including: material selection; screw placement; the effect of the fracture pattern; and the bone-plate offset. We demonstrate that device selection is not straight-forward as many of the variables influence one-another and an identically configured device can perform very differently depending upon the fracture pattern. Finally, we summarise the influence of some of the key parameters and the influence this can have on the fracture healing environment and the stresses within the plate in a flowchart.

Oblique Terminal Screw Placement: Does It Improve Fixation in Plate Osteosynthesis?

The technique of placing an oblique screw in the terminal hole of a plate to increase screw pullout strength is widely taught in the operating room. The origin of this technique is unclear; however, it may have been used simply as a means to identify radiographs and misinterpreted to have some biomechanical benefit. The objective of this study was to measure the structural effect of oblique terminal screw placement (OTSP) during plate osteosynthesis. Foam blocks and limited contact dynamic compression plates and screws were used along with a custom fixture device. The terminal screw was placed in either an oblique (30-degree angle outward) or perpendicular fashion. A load was applied perpendicular to the plate in cantilever bending until failure. The oblique screw construct was significantly weaker than the perpendicular screw construct (399N vs. 465N, P < 0.001), independent of the block of material used. Post hoc analysis showed that the screw angle (P < 0.001) was a significant determinant of the load required to cause screw pullout. OTSP led to a decrease in pullout strength compared with a perpendicular screw in a deformable foam medium similar in density to osteoporotic bone. In patients with poor bone quality, OTSP may create a suboptimal fracture fixation construct.

Failure of fracture fixation in osteoporotic bone

This manuscript will provide an overview of how the age and osteoporosis related changes in mechanical properties of bone affect the stability of osteosynthesis constructs, both from a mechanical as well as from a clinical perspective. The manuscript will also address some of the principles of fracture fixation for osteoporotic fractures and discuss applications of osteoporotic fracture fixation at sites typically affected by fragility fractures, namely the distal radius, the proximal humerus, the femur and the spine. The primary aim of operative treatment in elderly individuals is the avoidance of immobilization of the patient. In selected cases conservative treatment might be required. Generally, choice of treatment should be individualized and based on the evaluation of patient-specific, fracture-specific and surgeon-specific aspects. The orthopaedic surgeon plays an essential role in enabling functional recovery by providing good surgery but a multidisciplinary approach is essential in order to support the patient to regain his/her quality of life after fragility fracture. Overall, the therapy of fractures in osteoporotic bone in the elderly requires a multidisciplinary therapeutic acute care concept including treatment of co-morbidities and correct choice of timing, and technique of the operative intervention.

Surgical Reconstruction of Charcot Foot Neuroarthropathy, a Case Based Review

Our case-based review focuses on limb salvage through operative management of Charcot neuroarthropathy of the diabetic foot. We describe a case, when a below-knee amputation was considered in a patient with chronic Charcot foot with a rocker-bottom deformity and chronic plantar ulceration. Conservative treatment failed. Targeted antibiotic therapy and operative management (Tendo-Achilles lengthening, resectional arthrodesis of Lisfranc and midtarsal joints, fixation with large-diameter axial screws, and plaster cast) were performed. On the basis of this case, we discuss options and drawbacks of surgical management. Our approach led to healing of the ulcer and correction of the deformity. Two years after surgery, we observed a significant improvement in patient’s quality of life. Advanced diagnostic and imaging techniques, a better understanding of the biomechanics and biology of Charcot neuroarthropathy, and suitable osteosynthetic material enables diabetic limb salvage.

Reasons why dynamic compression plates are inferior to locking plates in osteoporotic bone: a finite element explanation

While locking plate fixation is becoming increasingly popular for complex and osteoporotic fractures, for many indications compression plating remains the standard choice. This study compares the mechanical behaviour of the more recent locking compression plate (LCP) device, with the traditional dynamic compression plates (DCPs) in bone of varying quality using finite element modelling. The bone properties considered include orthotropy, inhomogeneity, cortical thinning and periosteal apposition associated with osteoporosis. The effect of preloads induced by compression plating was included in the models. Two different fracture scenarios were modelled: one with complete reduction and one with a fracture gap. The results show that the preload arising in DCPs results in large principal strains in the bone all around the perimeter of the screw hole, whereas for LCPs large principal strains occur primarily on the side of the screw proximal to the load. The strains within the bone produced by the two screw types are similar in healthy bone with a reduced fracture gap; however, the DCP produces much larger strains in osteoporotic bone. In the presence of a fracture gap, the DCP results in a considerably larger region with high tensile strains and a slightly smaller region with high compressive strains. These findings provide a biomechanical basis for the reported improved performance of locking plates in poorer bone quality.

Topical review: locking plate technology in foot and ankle surgery

The use of locking plate technology in foot and ankle surgery has increased over the last decade. Reported applications include fracture repair, deformity correction, and arthrodesis. There is limited evidence, however, to guide clinicians with regard to the appropriate and optimal use of this technology. This work aims to examine the current biomechanical and clinical evidence comparing locking construct technology to other forms of fixation in the field of foot and ankle surgery.

Surgeon perception of cancellous screw fixation

OBJECTIVES

The ability of surgeons to optimize screw insertion torque in nonlocking fixation constructs is important for stability, particularly in osteoporotic and cancellous bone. This study evaluated screw torque applied by surgeons during synthetic cancellous fixation. It evaluated the frequency with which screws were stripped by surgeons, factors associated with screw stripping, and ability of surgeons to recognize it.

METHODS

Ten surgeons assembled screw and plate fixation constructs into 3 densities of synthetic cancellous bone while screw insertion torque and axial force were measured. For each screw, the surgeon recorded a subjective rating as to whether or not the screw had been stripped. Screws were then advanced past stripping, and stripped screws were identified by comparing the insertion torque applied by the surgeon to the measured stripping torque.

RESULTS

Surgeons stripped 109 (45.4%) of 240 screws and did not recognize stripping 90.8% of the time when it occurred. The tendency to strip screws was highly variable among individual surgeons (stripping ranging from 16.7% to 83.3%, P < 0.0001) and did not correlate with synthetic bone density in the range tested (P = 0.186) nor with the ranking of surgeons as resident or attending surgeon (P = 0.437). Screws that were correctly recognized as stripped retained a mean 55.0% of maximum torque, less than when stripping was not recognized (79.6%, P = 0.005).

CONCLUSIONS

Surgeon perception is not reliable at preventing and detecting screw stripping at clinical torque levels in synthetic cancellous bone. Less aggressive insertion or standardized methods of insertion may improve the stability of nonlocking screw and plate constructs.

Management of pubic symphysis diastasis with locking plates: a report of 11 cases

INTRODUCTION

The optimal method of fixation of symphyseal disruptions in pelvic ring injuries and post-operative rehabilitation is still debated. Options include two-hole, multi-hole and multiplanar plates. Post-operative rehabilitation can range from non-weight bearing bilaterally to full weight-bearing with crutches. Locking symphyseal plates have recently been introduced. There is a paucity of literature evaluating their use in such injuries. We present the first clinical case series of symphyseal diastasis managed with locking plates.

METHODS

A retrospective analysis of a single centre case series between August 2008 and December 2011 was conducted. A total of 11 patients; 2 females and 9 males with a mean age of 42 years were included. The mean radiological follow up was 27 weeks. Radiological failure and need for revision were evaluated.

RESULTS

4 patients sustained their injury as a result of a motorcycle accident, 3 patients following a car accident, 2 fell from a height and 2 had crush injuries. 9 patients had other concomitant injuries. The mechanism of injury was classified as anterior-posterior compression injury in 6 patients, vertical shear in 4 patients and combined mechanism in 1 patient. 6 patients required posterior pelvic fixation. Patients were mobilised fully or partially weight bearing. One patient had a significant radiological failure. All patients were asymptomatic at last follow-up and none required revision surgery.

CONCLUSION

Our series represents the first published clinical series of patients with symphyseal diastasis managed with locking plates. We have found the use of locking plates across the pubic symphysis to be safe with low complication rates despite early weight bearing.

The effect of locked screw angulation on the biomechanical properties of the S.P.S. Free-Block plate

OBJECTIVES

Among the locked internal fixators is one denominated S.P.S. (Synthesis Pengo System) Free-Block, which was designed with a locking ring that allows the screw to be locked and positioned obliquely. Due to the paucity of biomechanical studies on this system, the present work aimed to evaluate the influence of locked screw angulation on the resistance of the S.P.S. Free-Block plate.

METHODS

Forty synthetic bone cylinders with 10 mm fracture gap were used. Forty seven-hole 3.5 mm stainless steel plates (two AO-like dynamic compression holes and five locked holes) were assembled according to the orientation of the locked screws: monocortical screws were positioned at 90º to the long axis of the cylinder (Group 1), and monocortical screws were positioned at 70º to its cylinder long axis (Group 2). In both groups, AO-like dynamic compression hole screws were positioned bicortically and neutrally. For each group, six specimens were tested until failure, three in bending and three in compression, to determine the loads for fatigue testing. Subsequently, for each group, 14 specimens were tested for failure - seven by bending and seven in compression.

RESULTS

No significant failure differences were observed between Groups 1 and 2 under static-loading or fatigue test.

CLINICAL SIGNIFICANCE

In a fracture gap model the orientation of the locked monocortical screws did not show any influence on the mechanical performance of the S.P.S. Free-Block to tests of axial compression and four-point bending.

Peripheral and central bone mineral density in Charcot's neuroarthropathy compared in diabetic and nondiabetic populations

BACKGROUND

This prospective study was performed to compare calcaneal and lumbar bone mineral density (BMD) in individuals with and without diabetes mellitus. We compared bone density with the time from onset of Charcot's neuroarthropathy (CN) in patients with unilateral, nonoperative, reconstructive-stage CN. The final purpose was to investigate the role that sex, age, and serum vitamin D level may have in osseous recovery.

METHODS

Thirty-three individuals were divided into three groups: controls and patients with diabetes mellitus with and without CN. Peripheral instantaneous x-ray imaging and dual-energy x-ray absorptiometry were performed.

RESULTS

The calcaneal BMD of patients with diabetes mellitus and CN was lower than that of the control group (P < .01) but was not significantly lower than that of patients with diabetes mellitus alone. There was no statistically significant difference in lumbar T-scores between groups. Women demonstrated lower BMD than did men (P = .02), but patients 60 years and older did not demonstrate significantly lower BMD than did patients younger than 60 years (P = .135). A negative linear relationship was demonstrated between time and BMD in patients with CN.

CONCLUSIONS

The results of this study suggest that lumbar BMD does not reflect peripheral BMD in patients with diabetes mellitus and reconstructive-stage CN. This study has clinical implications when reconstructive osseous surgery is planned in patients with CN.

Failure of locked design-specific plate fixation of the pubic symphysis: a report of six cases

OBJECTIVES

Physiological pelvic motion has been known to lead to eventual loosening of screws, screw breakage, and plate breakage in conventional plate fixation of the disrupted pubic symphysis. Locked plating has been shown to have advantages for fracture fixation, especially in osteoporotic bone. Although design-specific locked symphyseal plates are now available, to our knowledge, their clinical use has not been evaluated and there exists a general concern that common modes of failure of the locked plate construct (such as pullout of the entire plate and screws) could result in complete and abrupt loss of fixation. The purpose of this study was to describe fixation failure of this implant in the acute clinical setting.

DESIGN

Retrospective analysis of multicenter case series.

SETTING

Multiple trauma centers.

PATIENTS

Six cases with failed fixation, all stainless steel locked symphyseal plates and screws manufactured by Synthes (Paoli, PA) and specifically designed for the pubic symphysis, were obtained from requests for information sent to orthopaedic surgeons at 10 trauma centers. A four-hole plate with all screws locked was used in 5 cases. A six-hole plate with 4 screws locked (two in each pubic body) was used in one.

INTERVENTION

Fixation for disruption of the pubic symphysis using an implant specifically designed for this purpose.

MAIN OUTCOME MEASUREMENTS

Radiographic appearance of implant failure.

RESULTS

Magnitude of failure ranged from implant loosening (3 cases), resulting in 10-mm to 12-mm gapping of the symphyseal reduction, to early failure (range, 1-12 weeks), resulting in complete loss of reduction (3 cases). Failure mechanism included construct pullout, breakage of screws at the screw/plate interface, and loosening of the locked screws from the plate and/or bone. Backing out of the locking screws resulting from inaccurate insertion technique was also observed.

CONCLUSIONS

Failure mechanisms of locked design-specific plate fixation of the pubic symphysis include those seen with conventional uniplanar fixation as well as those common to locked plate technology. Specific indications for the use of these implants remain to be determined.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

The insertional torque of a pedicle screw has a positive correlation with bone mineral density in posterior lumbar pedicle screw fixation

In patients with osteoporosis there is always a strong possibility that pedicle screws will loosen. This makes it difficult to select the appropriate osteoporotic patient for a spinal fusion. The purpose of this study was to determine the correlation between bone mineral density (BMD) and the magnitude of torque required to insert a pedicle screw. To accomplish this, 181 patients with degenerative disease of the lumbar spine were studied prospectively. Each underwent dual-energy x-ray absorptiometry (DEXA) and intra-operative measurement of the torque required to insert each pedicle screw. The levels of torque generated in patients with osteoporosis and osteopenia were significantly lower than those achieved in normal patients. Positive correlations were observed between BMD and T-value at the instrumented lumbar vertebrae, mean BMD and mean T-value of the lumbar vertebrae, and mean BMD and mean T-value of the proximal femur. The predictive torque (Nm) generated during pedicle screw insertion was [-0.127 + 1.62 × (BMD at the corresponding lumbar vertebrae)], as measured by linear regression analysis. The positive correlation between BMD and the maximum torque required to insert a pedicle screw suggests that pre-operative assessment of BMD may be useful in determining the ultimate strength of fixation of a device, as well as the number of levels that need to be fixed with pedicle screws in patients who are suspected of having osteoporosis.

Evidence for success with locking plates for fragility fractures

Fixation of fragility fractures with plates and screws often results in loss of fixation and need for revision surgery. Locking plates and screw were introduced to improve fixation of fragility fractures and have been in use for a decade. This review was conducted to compile evidence that locking plates and screws improve fixation of fragility fractures. A search of PubMed was performed to identify biomechanical studies as well as clinical series of fragility fractures treated with locking plates. Biomechanics papers had to use models of osteoporotic bone and had to directly compare locking plates with traditional plates. Clinical studies included case series in which locking plates were applied to elderly patients with fractures of the proximal humerus and periprosthetic distal femur fractures. Most studies are retrospective case series. Locking plates lead to greater stability and higher loads to failure than traditional plates. When applied to proximal humerus fractures, uncomplicated healing occurs in 85% of patients. Constant and Dash scores approach normal values. For distal femoral periprosthetic fractures, union rates of 75% are reported with a malunion rate of 10%. Early evidence suggests that locking plates improve results of treatment of proximal humerus fractures and distal femoral periprosthetic fractures in the elderly. Loss of fixation is associated with failure to achieve stability at the fracture site. Principles of fracture fixation in osteoporotic bone defined prior to the introduction of locking plates should still be applied.

Pullout strength of pedicle screws with cement augmentation in severe osteoporosis: a comparative study between cannulated screws with cement injection and solid screws with cement pre-filling

Background

Pedicle screws with PMMA cement augmentation have been shown to significantly improve the fixation strength in a severely osteoporotic spine. However, the efficacy of screw fixation for different cement augmentation techniques, namely solid screws with retrograde cement pre-filling versus cannulated screws with cement injection through perforation, remains unknown. This study aimed to determine the difference in pullout strength between conical and cylindrical screws based on the aforementioned cement augmentation techniques. The potential loss of fixation upon partial screw removal after screw insertion was also examined.

Method

The Taguchi method with an L₈ array was employed to determine the significance of design factors. Conical and cylindrical pedicle screws with solid or cannulated designs were installed using two different screw augmentation techniques: solid screws with retrograde cement pre-filling and cannulated screws with cement injection through perforation. Uniform synthetic bones (test block) simulating severe osteoporosis were used to provide a platform for each screw design and cement augmentation technique. Pedicle screws at full insertion and after a 360-degree back-out from full insertion were then tested for axial pullout failure using a mechanical testing machine.

Results

The results revealed the following 1) Regardless of the screw outer geometry (conical or cylindrical), solid screws with retrograde cement pre-filling exhibited significantly higher pullout strength than did cannulated screws with cement injection through perforation (p = 0.0129 for conical screws; p = 0.005 for cylindrical screws). 2) For a given cement augmentation technique (screws without cement augmentation, cannulated screws with cement injection or solid screws with cement pre-filling), no significant difference in pullout strength was found between conical and cylindrical screws (p >0.05). 3) Cement infiltration into the open cell of the test block led to the formation of a cement/bone composite structure. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, whereas the composite remained well bonded to the screws. This result implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. 4) The back-out of the screw by 360 degrees from full insertion did not decrease the pullout strength in any of the studied cases. 5) Generally, larger standard deviations were found for the screw back-out cases, implying that the results of full insertion cases are more repeatable than those of the back-out cases.

Conclusions

Solid screws with retrograde cement pre-filling offer improved initial fixation strength when compared to that of cannulated screws with cement injection through perforation for both the conically and cylindrically shaped screw. Our results also suggest that the fixation screws can be backed out by 360 degrees for intra-operative adjustment without the loss of fixation strength.

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.