Screw Purchase Research Articles

Effect of screw placement on fixation in the humeral head

The objectives of this study were (1) to determine the most advantageous screw locations within the humeral head when plate and screw fixation is to be used and (2) to determine the effect of positioning the screw tip abutting the subchondral bone. Ten paired humeral heads were harvested with a monoplanar cut through the anatomic neck. Through use of a standardized template, 7 holes were drilled and tapped in each specimen for insertion of 6.5-mm fully threaded cancellous screws perpendicular to the plane of the cut. Paired specimens were randomized into 2 groups, one with the screw purchase in central cancellous bone and the other with the screw purchase up to the subchondral bone. Each screw was pulled out axially at a displacement rate of 10 mm/min through use of a servohydraulic testing machine. The length of thread purchase, position within the head, and screw pull-out load to failure were recorded. The normalized pullout force to failure was calculated by dividing absolute pullout force to failure by length of screw purchase. Data were analyzed by means of a 2-way repeated measures analysis of variance and post hoc Student-Newman-Keuls test. The central position had a significantly higher absolute pullout force to failure than all other sites (P < .05). By virtue of the humeral head shape, the central position also had a significantly greater length of screw purchase than all other positions (P < .05). The central position had a significantly higher relative pullout force to failure than all other positions (P < .05). Subchondral bone abutment positioning improved both the absolute and the relative pullout forces to failure (P < .05). When screws and plates are used in open reduction and internal fixation of a proximal humerus fracture, a major mode of failure is loss of fixation within the humeral head. On the basis of this study, optimal screw purchase with respect to bone fixation can be achieved by including screws located in the center of the humeral head in the subchondral abutment position. To minimize screw fixation failure, the anterosuperior position should be avoided. The pattern of distribution of the relative pullout force as measured in this study is consistent with previous observational studies of patterns of trabecular density within the humeral head.

Efficacy of anterior odontoid screw fixation in elderly patients with Type II odontoid fractures

Type II odontoid fractures are the most common trauma-related dens fracture. Although Type III odontoid fractures have a high union rate when external immobilization is applied, Type II fractures are associated with high rates of nonunion, particularly in elderly patients and those with posteriorly displaced fractures or fractures displaced by more than 6 mm. Because elderly patients may not also tolerate external immobilization in a halo vest, alternative techniques should be explored to identify a method for managing these higher-risk patients. In this study the authors examine the efficacy of anterior odontoid screw fixation in a high-risk group of 10 elderly patients (> 65 years of age) treated for Type II odontoid fractures. A retrospective review of all patients with Type II odontoid fractures treated at two institutions between September 1997 and March 2000 was performed. Demographic data, neurological examination, fracture type and degree of displacement, treatment method, and outcome data were examined at discharge. Ten patients older than 65 years who had sustained a trauma-related odontoid fracture and had undergone an anterior odontoid screw placement procedure were retrospectively reviewed. Fracture displacement (mean 6.6 mm) was observed in all but one patient, and in seven there were posteriorly displaced fractures. Seven were successfully treated with anterior screw fixation and external orthosis alone; in one patient in whom poor intraoperative screw purchase had been observed, the fracture healed after undergoing halo vest therapy. Only one patient was shown to develop a nonunion requiring a subsequent posterior fusion procedure. Odontoid screw fixation can be safely performed in elderly patients, and frequent bone union is demonstrated. However, osteopenia may preclude adequate screw fixation in some patients.

New Technique for Treatment of Unstable Distal Femur Fractures by Locked Double-Plating: Case Report and Biomechanical Evaluation

A comminuted, intra-articular distal femur fracture was surgically treated by the authors with a locked, double-plating technique because fixation stability could not be initially achieved by using a standard double-plating technique. The purpose of this study was to determine biomechanically whether a locked double-plate construct would enhance fixation stability compared with a nonlocked double-plate construct. Six matched pairs of mildly osteopenic femurs were selected and all had a reproducible intra-articular fracture pattern created. Each pair underwent fixation with either a double-plating construct or a locked, double-plating construct that was randomly assigned. The instrumented femurs were then mechanically tested in several loading modes to determine fixation stability. After initial testing, specimens were cyclically loaded and retested for stability. The locked, double-plating construct provided significantly greater fixation stability than the standard double-plating construct in precycling and postcycling biomechanical testing. The technique described is particularly applicable for severely comminuted fractures of the distal femur and fractures in osteopenic bone with poor screw purchase. It offers a simple alternative for enhancing fixation stability, which avoids the potential complications of methylmethacrylate-enhanced screw fixation.

Peroperative determination of safe superior transarticular screw trajectory through the lateral mass.

Computerized anatomic reconstruction of the dry axis vertebra was performed to determine radiologic guidelines for safe superior transarticular screw trajectory. To reconstruct the transarticular screw trajectory, using computer-aided design techniques, and develop a technique that provides real-time intraoperative guidance during screw placement. A recent osteometric study of 50 dry specimens of the axis noted significant vertebral artery groove anomalies in 22% of specimens. There are presently no anatomic or radiologic guidelines to help surgeons avoid an enlarged vertebral groove, despite the fact that a safe screw trajectory through the lateral mass is primarily dependent on the its depth and the internal height of the lateral mass. Using computer-aided design techniques, we re-analyzed the vertebral grooves of 50 dry specimens and mapped minimum and corrected safe superior trajectories for any given depth of this groove. This knowledge was extrapolated to spiral computed tomographic scan data, which was used to develop the clinical method for safe superior trajectory. Real-time fluoroscopy was used to apply the method intraoperatively. Internal height less than 2.1 mm or values less than 0.85 for the ratio of the mean internal height over the mean vertebral groove depth would result in unacceptable risk to vertebral artery injury and improper screw purchase. With every 0.5-mm increase in groove depth, the angle of trajectory increases by 1 degree at a pedicle length of 30 mm. There is an inverse linear relation between the superior angle of trajectory and the pedicle length (2 degrees = 5 +/- 0.5 mm). Screw diameter-dependent trajectory correction is required (3.5 mm = 7 degrees). Before atlantoaxial transarticular surgery, vertebral groove depth should be evaluated and a safe screw trajectory angle should be plotted to determine anatomic suitability. This trajectory angle can be used with intraoperative real-time fluoroscopy to guide the surgeon during screw insertion.

Posterior Cervical Arthrodesis and Stabilization: An Early Report Using a Novel Lateral Mass Screw and Rod Technique

OBJECTIVE: Posterior cervical arthrodesis and stabilization with lateral mass plates is a biomechanically sound construct in multiple planes of motion. It is reproducible and especially useful when the posterior elements are missing or fractured. Unfortunately, it is difficult to use in patients with severe degenerative spondylosis because the plate is malleable only in the sagittal plane and the screw positions are dictated by the plate's entry holes. METHODS: A novel system of lateral mass screws that can be positioned before placement of a lateral construct was used in nine patients. Their outcomes as well as the technical applications of this system were reviewed. RESULTS: A total of 52 screws were placed in nine patients who underwent posterior cervical arthrodesis with the Cervifix system (Synthes USA, Paoli, PA). Diagnoses included trauma in four patients, degenerative spondylosis in three, and tumor in two. Rods were molded individually according to the patient's anatomy. Compression, distraction, and lateral rotation, if indicated, were performed. Follow-up averaged 36 weeks. Lateral and anteroposterior radiographs, obtained at progressive intervals, revealed excellent fixation and screw purchase without pull-out. There were no cases of spinal cord, nerve root, or vertebral artery injury. CONCLUSION: The Cervifix system accommodates variation in anatomic size and spacing of the lateral masses, potentiating precise screw placement. The rods can be molded in multiple planes, and selective application of compressive, distractive, or lateral rotatory forces is allowed. The system is very straightforward and simple to use, and we have had good success without pseudarthrosis or complications from screw placement in our series.

Fine Morphological Assessment of Stripped Screw Sites in Cortical Bone

SummaryUnintentional over-tightening of orthopaedic screws resulting in loss of screw purchase (“screw stripping”) is a potential complication during fracture fixation. This report describes the microstructural and radiographic effects on bone caused by stripping a 2.7 mm orthopaedic screw, and subsequent replacement in these stripped screw sites with either a 3.5 mm cortical or a 4.0 mm cancellous orthopaedic screws in both tapped and untapped holes. Over-tightening of a 2.7 mm screw resulted in shearing of the bone directly engaged by the screw threads. Additional fractures occurred in surrounding bone, primarily superficial subperiosteal saucer fractures in the cis cortex and to a lesser degree, in the subendosteum of the trans cortex. Bone damage was consistently more severe in the cis cortex. Salvaging stripped screw sites by insertion of a 3.5 mm or 4.0 mm screw resulted in additional fracturing and displacement, which was more severe in the untapped sites. Decreased holding power of screws placed in stripped screw sites could be attributed to collateral structural bone damage occurring during screw stripping and replacement screw application.Over-tightening of orthopaedic screws until mechanical failure (“screw stripping”) results in damage to cortical bone which extends beyond the outer thread diameter of the screw. Damage, most often subperiosteal saucer fractures, was greater in the cis cortex. The subperiosteal saucer fractures decreased the functional length of screw engagement of the larger replacement screws in the cortical bone, which may account for decreases in expected holding power.

Computed tomographic evaluation of the internal structure of the lateral mass of the lower cervical spine.

Axial computed tomography scans with a slice thickness of 2 mm taken from 12 cervical spines were used to study the internal structure of the lateral mass. Images representing the middle of zone I (Heller's classification) and the top of zone III were analyzed. The measurement of zone I involved anterior cortex thickness (ACT) while measurements of zone III included ACT, lateral cortex thickness (LCT), posterior cortex thickness (PCT), lateral mass thickness (LMT), and lateral mass width (LMW). The percentage of the ACT and PCT with respect to the LMT (ACT/LMT and PCT/LMT) were calculated. Results showed the average ACT in zone I ranged from 1.6 to 1.8 mm. In zone III, the average LMT and LMW ranged from 8 to 9 mm and 13 to 15 mm, respectively. The smallest LMT was found at C7. The average ACT and PCT for all levels ranged from 1.8 to 2 mm. The ACT with respect to the LMT (ACT/LMT) was approximately 17% to 19% for C3 to C5 and C7, and 15% for C6 separately. The PCT with respect to the LMT (PCT/LMT) was approximately 16% to 18% for C3 to C6, and 20% for C7. These results show the ventral cortex of the lateral mass is relatively thicker and support the concept of bicortical screw purchase.

Unilateral posterior atlantoaxial transarticular screw fixation.

Bilateral posterior C 1-2 transfacet screw placement with associated posterior bone graft wiring is the accepted treatment for patients with atlantoaxial instability. This technique was modified to treat 19 patients with atlantoaxial instability and unilateral anomalies that prevented placement of a screw across the C1-2 facet. In these cases, a single contralateral transarticular screw was placed in conjunction with interspinous bone graft wiring to avoid neural or vertebral artery injury and to provide C1-2 stability. Postoperatively, all 19 patients were placed in Philadelphia collars (mean immobilization 8 weeks, range 6-12 weeks). Unilateral C1-2 facet screw fixation was needed for the following reasons: a high-riding transverse foramen of the C-2 vertebra present in 13 patients (left side in eight, right side in five), poor screw purchase in two (left side in both), screw malposition in one (left side), severe degenerative arthritis in one (right side), neurofibroma in one (right side), and fracture of the C-1 lateral mass in one (left side). Six weeks postsurgery, one patient presented with a broken screw and required occipitocervical fusion with a Steinmann pin and wire cable from the occiput to C-3 to achieve solid fusion. Solid fusions were achieved in the other 18 patients (mean follow-up period 31 months, range 14-54 months); there was no delayed screw breakage, wire breakage, or spinal instability. There were no operative or postoperative neurological or vascular complications. The authors' experience demonstrates that unilateral C1-2 facet screw fixation with interspinous bone graft wiring is an excellent alternative in the treatment of atlantoaxial instability when bilateral screw fixation is contraindicated.

Assessment of Primary and Salvage Lateral Mass Screw Insertion Torque in a Cadaveric Model

This study was performed to determine the insertion torques of three types of lateral mass screws and to explore their relationship to bone mineral density (BMD). The peak insertion torque of primary articular screws was measured, and the holes were then stripped by overtightening. Larger diameter salvage screws were placed in the stripped holes, and peak insertion torque was measured. Peak primary screw insertion torque and BMD were positively correlated (r = 0.48, p = 0.00001). The ratio of salvage to primary screw insertion torque varied significantly among the systems (p = 0.0003). The positive correlation between BMD and primary screw peak insertion torque confirms the importance of bone density for satisfactory screw purchase. The large variation in the ratio of salvage to primary screw insertion torque among systems may be due to differences in primary and salvage screw design.

Assessment of Primary and Salvage Lateral Mass Screw Insertion Torque in a Cadaveric Model

This study was performed to determine the insertion torques of three types of lateral mass screws and to explore their relationship to bone mineral density (BMD). The peak insertion torque of primary articular screws was measured, and the holes were then stripped by overtightening. Larger diameter "salvage" screws were placed in the stripped holes, and peak insertion torque was measured. Peak primary screw insertion torque and BMD were positively correlated (r = 0.48, p = 0.00001). The ratio of salvage to primary screw insertion torque varied significantly among the systems (p = 0.0003). The positive correlation between BMD and primary screw peak insertion torque confirms the importance of bone density for satisfactory screw purchase. The large variation in the ratio of salvage to primary screw insertion torque among systems may be due to differences in primary and salvage screw design.

Factors affecting the pullout strength of cancellous bone screws.

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

Interphalangeal joint arthrodesis with oblique placement of an AO lag screw

A damaged interphalangeal (IP) joint may be treated by fusion. Arthrodesis should leave the joint at the most functional angle and give sound bony union in the shortest possible time, maintaining maximum proximal and distal joint motion. The lateral oblique placement of a single AO lag screw, proximal to distal, achieves these aims. This method gives sufficient proximal bone for screw purchase and better control of the desired angle of fusion. This technique has led to fusion in 22 of 23 joints (96%), taking an average of 8.2 weeks.

Treatment of intra-articular fractures of Os calcis with open reduction, internal fixation and bone graft

Surgical Principles The subtalar joint is a major weight bearing joint in the lower limb. Displaced intra-articular fractures of the os calcis should be treated with anatomical reduction and stable internal fixation to allow early mobilization. Intra-articular fractures of the os calcis lead to loss of joint congruity, and impaction of the cancellous bone. Bone grafting, together with stable internal fixation, allow early mobilization and weight bearing after surgery. The operation is done with the lateral approach which allows direct access to the fracture site. The subchondral bone, the sustentaculum tali and the medio-inferior part of the os calcis provide good bone stock for the purchase of screws. After reduction and fixation of the depressed fragments, the space is filled with cortico-cancellous grafts. The lateral wall of the os calcis is buttressed with a plate. Postoperatively, passive mobilization is started early, walking with a weight relieving calliper lasts for the first six weeks and graduated weight bearing is started on the seventh week.

Pedicle screw pullout strength. Correlation with insertional torque.

This study was designed to correlate several parameters regarding pedicle screw bone/metal interface strength. The insertional torque measured during tapping and placement of pedicle screws was correlated with the bone mineral density of the vertebral body, the dimensions of the pedicle, the method of preparation of the pedicle, and the amount of load and number of cycles to failure of the bone/metal interface. Thirty human cadaveric lumbar vertebrae were instrumented with 6.5-mm pedicle screws. The maximum torque achieved during insertion was digitally recorded. A cyclic pedicle screw pullout test was performed. A linear correlation existed between both the insertional torque when tapping or when inserting a screw and the number of cycles to ultimate pedicle screw pullout. An inverse linear relationship was found between the pedicle width and cycles to failure. There was no linear correlation found when comparing the number of cycles to failure to bone mineral density. These findings suggest that insertional torque is a good predictor of bone-metal interface failure. Bone mineral density of the vertebral body was less effective as a predictor of failure. Smaller pedicle width correlated with increased insertional torque and cycles to failure. This may explain why patients with osteoporosis on radiography may still obtain stable fixation with pedicle screws. Other factors, such as pedicle dimensions and shape, affect screw purchase as much as vertebral body bone density. Insertional torque less than 4.0 inch-pounds led to early pedicle screw pullout. This study forms the basis for the authors' clinical use of an instrumented torque screwdriver to measure insertional torque in the operating room.

Atlanto-axial stabilization with posterior transarticular screw fixation: technical description and report of 22 cases.

Magerl's technique of combining C1-C2 posterior screw fixation with a supplemental bone-wire fusion has been advocated for the management of atlanto-axial instability. Between October 1990 and August 1992, a modification of this technique was used in the treatment of 22 patients with this disorder. In the absence of spinal deformity or neoplastic disease, screw fixation and bony fusion were used alone without associated wiring, thus avoiding the risk of neural injury resulting from the sublaminar passage of wire and the retrodisplacement of ventral structures. Patient ages ranged from 30 months to 80 years; follow-up ranged from 5 to 27 months, with a mean of 14.9 months. The causes of the instabilities were as follows: eight cases of nonunion of Type II odontoid fracture, four cases of rheumatoid arthritis, three cases of tumor, two cases of ligamentous instability, two cases of pseudoarthrosis after bone-wire fusion, two cases of halo noncompliance, and one case of Os odontoideum. All 20 patients who underwent fusion were placed in a Philadelphia collar for 12 weeks. Nineteen of 20 (95%) patients achieved solid fusion. Twenty-one of 22 (95%) had significant reduction in preoperative pain. No patient developed myelopathy or bulbar findings. The one intraoperative complication was an inability to achieve secure screw purchase on one side that required unilateral screw placement with a Gallie fusion-using cable. Postoperative complications included one patient with a superficial wound infection that resolved after local debridement and antibiotics and suboccipital numbness in two patients. Progression of spinal deformity, screw pullout or breakage, and neurological or vascular complications did not occur.(ABSTRACT TRUNCATED AT 250 WORDS)

Anterior Cervical Plate Fixation with the Titanium Hollow Screw Plate System A Preliminary Report

Morscher, of Switzerland, has developed an anterior cervical spine plate system (THSP) that does not require screw purchase of the posterior cortex. This design eliminates potential neurologic complications usually associated with the anterior plate system, but maintains the mechanical advantages of internal fixation. The authors reviewed 13 consecutive patients in whom the THSP system was applied. Indications for the use of this device included acute trauma in three patients, trauma of more than 6 weeks' duration in five patients, and spondylosis in five patients. Fifteen plates and 58 screws were placed, with no screws purchasing the posterior cortex. Postoperative immobilization varied from no immobilization to four-poster brace. With a mean follow-up of 13 months, all 13 patients went on to fusion. One patient had screws placed in the disc rather than in bone and went on to malunion. In all other patients, radiographs did not demonstrate screw migration, screw-bone lucency, graft dislodgement, or malunion. No patient suffered neurologic injury as a result of this device. The THSP system facilitates reliable fusion with minimal complications. Its use should be considered in multilevel anterior spine defects, posttraumatic cervical kyphosis, and cervical fractures with posterior disruption requiring anterior fusion.

Morphometric analysis of the thoracolumbar and lumbar pedicles, anatomo-radiologic study.

The geometric properties of 380 vertebral pedicles, ranging from T6 to L5, were analysed. Measurement were made directly from the specimens as well as from roentgenograms. The parameters considered were the horizontal and vertical pedicle diameters, pedicle angles in the transverse and sagittal planes, and the transverse and anteroposterior widths of the spinal canal and vertebral body. In addition, the length of the pedicle and the length of the pedicle including the vertebral body to the anterior cortex were measured along the pedicle axis and in a line parallel to the midline of the vertebral body. The smallest horizontal and vertical pedicle diameters were found at vertebral levels from T6 to T10. The correlation between pedicle widths and screw dimensions is obvious. In the transverse plane, the pedicle angle diverged from the vertebral body at all levels, except at T12. In the sagittal plane, the pedicles were angled cephalad from T6 to L3 and slightly caudally at L5. Knowledge of the length of the pedicle to the anterior vertebral body cortex is very important for safe screw purchase. At all levels, with the exception of T12, this length was found to be significantly greater along the pedicle axis than along a line parallel to the midline of the vertebral body.

Semitubular Blade Plate for Fixation in the Proximal Humerus

Rigid internal fixation is frequently difficult to obtain in the proximal humerus, as osteoporotic bone and small fracture fragments may preclude firm purchase of plates and/or screws. We describe our clinical results using a semitubular plate, fashioned into a blade plate device, for fixation of four displaced fractures and three osteotomies of the proximal humerus. All patients were clinically and radiographically united by 4 months postoperatively and had a functional range of motion. The strength of this fixation was compared to that of an AO "T" plate in an oblique subcapital osteotomy model using 10 matched pairs of human humeri. No statistically significant difference could be demonstrated between the two fixation methods with regard to load to failure, yield load, energy absorbed to failure, or stiffness. As a consequence of these studies, we believe the semitubular blade plate expands the options available for challenging fixations in the proximal humerus.

Nonunion of the Humerus

Five patients with humeral nonunions were treated by open reduction and internal fixation with compression plating and adjunctive methylmethacrylate inserted by use of several techniques to gain secure screw purchase in markedly osteoporotic bone. The rigid fixation was supplemented in each case with cortical-cancellous bone grafting. Postoperative abduction splinting was applied in each case. The average follow-up examination was 24 months, with all five nonunions having healed at an average of five months postoperation. The shoulder arc of motion averaged 120 degrees, and the elbow arc of motion averaged 125 degrees.