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00007632-199806150-0001500007632_1998_23_1374_polly_shimswhat_12miscellaneous-article< 105_0_17_5 >Spine© Lippincott-Raven Publishers.Volume 23(12)15 June 1998p 1374–1379Revision Pedicle Screws: Bigger, Longer Shims‐What Is Best?[Biomechanics]Polly, David W. Jr MD, LTC, MC*; Orchowski, Joseph R. MD, CPT, MC*; Ellenbogen, Richard G. MD†‡From the *Orthopaedic Surgery Service and the †Neurosurgery Service, Walter Reed Army Medical Center, Washington, DC; and the ‡Department of Neurological Surgery, University of Washington, Seattle, Washington.Funded, in part, by the Department of Clinical Investigations, Walter Reed Army Medical Center, Washington, DC.The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.Acknowledgment date: November 23, 1996.First revision date: April 3, 1997.Acceptance date: July 22, 1997.Device status category: 11.Address reprint requests to: David W. Polly, Jr, MD; Director, Spine Surgery; Orthopaedic Surgery Service; Walter Reed Army Medical Center; Washington, DC 20307‐5001.AbstractStudy Design. To evaluate the effect of change in screw dimensions and hole augmentation in pedicle screw revisions, the insertional torque was determined, and results were compared with those in control specimens in an in vitro study using cadaveric thoracolumbar spines.Objectives. To determine the best method of salvage for failed pedicle screws, by evaluating the insertional torque after placing a larger diameter or longer screw into a stripped hole. Use of a shim and use of larger and longer screws were also investigated. Finally, the effect on insertional torque of simply removing and replacing a pedicle screw in its original hole was investigated.Summary of Background Data. The effects of using bigger or longer screws and shims to salvage failed pedicles have been studied. The interaction between how much larger, how much longer, and inserting with or without shims, has not been well studied. Optimizing reinsertional torque through the use of bigger screws risks exceeding the pedicle capacity. Using longer screws risks violation of the anterior vertebral body, thereby placing the great vessels and viscera at risk. By knowing the relative contribution of increase in length and diameter, the surgeon can optimize the risk‐benefit ratio.Methods. Eight cadaveric spines from T10 to S1 were harvested. The specimens underwent radiographic screening and bone densitometry. A modified Latin square randomization was designed to evaluate the screw diameters and lengths. Each pedicle was its own control. A 35‐ × 6.5‐mm screw was used as a control. Test screws were placed after pedicle screw hole failure was achieved and documented by stripping. For the test screws, the diameters were increased by 1 mm and 2 mm, the lengths were increased by 5 mm and 10 mm. Shims were added randomly. The peak insertional torque was measured for each control screw and test screw placement. In addition, during each screw placement, the screw was removed and replaced to determine the effect.Results. Insertional torque, after the pedicle screw is removed and replaced in the same hole, was decreased by 34% (P < 0.000005). Increasing the diameter of the salvage screw by 2 mm caused the insertional torque to be increased by 8.4% of the original. Increasing the length of the screw did not improve the salvage screw insertional torque. There was an interaction effect for the 1‐mm increase in diameter and the increase in length. At this diameter, increasing the length had a significant effect (P = 0.009) on the salvage torque. Using a shim created no improvement in salvage insertional torque (P = 0.77). There was a poor linear correlation between torque and bone mineral density (r = 0.18) in these osteoporotic specimens.Conclusions. Removing and replacing a pedicle screw in its original hole substantially decreases its mechanical fixation. For pedicle salvage, increasing the diameter causes the greatest restoration of strength. Shims had no effect in pedicle salvage in osteoporotic specimens.Treatment of spinal deformity, degenerative disease, trauma, and tumors often require internal fixation to provide temporary rigid stabilization while allowing the body to achieve the long term biologic solution of fusion. The more rigid the fixation, the more rapid and successful the fusion.21 Since the early 1980s, bone screws placed through the pedicle have become a common method used to attach internal fixation to the spine. In prospective randomized studies, Zdeblick23 and Lorenz et al10 both showed that fusion with instrumentation using pedicle screws was more successful than fusion without instrumentation. Pedicle screws provide the most rigid fixation currently available.4 Within a construct, pedicle screws achieve more rigid fixation than hooks or sublaminar wires.8Because pedicle screws are used more frequently, there are a small number of cases of screw failure. In patients in whom a solid fusion has not been achieved, pedicle screw salvage and revision may be necessary. Salvaging screw purchase in any bone can only be accomplished by using a larger diameter screw, by using a longer screw, by using a screw both larger in diameter and longer, by augmenting the failed hole, or by placing the screw in a new site. Pedicle screws have substantial anatomic constraints and risks for each of the first four options noted above. The final option can only rarely be used unless another level is added to the fusion. The risk of using a larger screw is pedicle fracture with possible neural injury, especially if the medial or inferior pedicle wall is violated. The risk of using a longer screw is cortical violation of the anterior vertebral body with risk of vascular or visceral injury. The risk of hole augmentation with substances such as polymethylmethacrylate (PMMA) include extravasation with neurologic injury from direct compression or thermal effects. Therefore, it is important that the surgeon be able to weigh the relative risks and benefits of the various forms of salvage.Obviously, salvage cases have great variability, and it makes direct comparison of these techniques difficult clinically. Therefore, testing is typically conducted in a laboratory setting using anatomic tissue. Most investigations in which the mechanical strength of pedicle screw placement is measured use screw axial pullout strength as their dependent variable. Recently, there have been a number of articles correlating pullout strength with insertional torque.3,5,13,15,17,24 These investigators found that a linear correlation exists between the insertional torque and the pullout strength. The linear correlation coefficient for these studies ranges from 0.83 to 0.925. This linear correlation is consistent with mechanical behavior of screws. For example, for power screws, the torque applied is a function of the axial force created and the screw design parameters.16Okuyama et al13 found that the insertional torque could predict mechanical stability. Others also supported this finding.3,5,13,15,17,24 Using torque as a predictor of mechanical strength is clinically relevant, because during pedicle screw placement the surgeon uses experience to gauge whether sufficient purchase has been achieved.5 The surgeon determines this by the feel of the necessary torque applied to tighten the screw. In the current investigation, the authors used the maximum torque created before seating the final thread as a measure of the mechanical strength of the pedicle screw placement. This technique provided a measurement of the mechanical purchase at the screw‐bone interface.3,13The purpose of this study is to investigate the relative improvement of insertional torque for salvage of pedicle screws using larger screws, longer screws, larger and longer screws, and hole augmentation with bone shims in human cadaveric thoracolumbar spines.Materials and MethodsEight human cadaveric spines were used. The age of the cadavers ranged from 42 to 92 years, with a mean of 77.1 years. There were four female and four male specimens. Vertebrae T10 through the sacrum were harvested. Soft tissues were dissected, leaving only the osteoligamentous column intact. The specimens were screened for tumors and fractures by radiographs. Each specimen underwent dual‐energy x‐ray absorptiometry (DEXA) with a bone densitometer (QDR‐2000; Hologic, Waltham, MA). Ages, gender, and bone mineral density of the specimens are shown in Table 1. The specimens were stored at −20 C until time of experimentation. They were thawed at room temperature for 18 hours before their use. All pedicle screws were placed by an experienced spine surgeon with extensive clinical experience in the use of pedicle screws. Gertzbein and Robbins6 showed that there is a substantial learning curve to placement of pedicle screws.Table 1. Specimen Age, Gender, and Bone Mineral DensityStainless steel bone screws were used (Sofamor‐Danek DYNA‐LOCK system; Memphis, TN). These screws were inserted according to a standard clinical protocol for pedicle screw placement.2,21,22 The hole was prepared by first breaching the cortex using an awl. The pedicle tract was then established to a depth of 35 mm using a blunt‐tipped ganglion knife pedicle probe. The hole was not tapped. For each vertebra, a standard 35‐ × 6.5‐mm pedicle screw was inserted to a standard depth of 35 mm. At this depth, the screw went through the pedicle and into the body, but it did not breech the anterior cortex. This was confirmed by visual inspection.To determine the mechanical strength of the screw‐bone interface, a digital torque wrench (Snap‐On, Kenosha, WI; range, 0‐10 inch‐pounds)24 was used to measure the maximum insertional torque required for the final 1 mm of screw insertion. Screw insertion was complete when the head of the screw was firmly seated on the surface of the bone and there were no remaining threads. Once the insertional torque for the control screw was obtained, the screw was backed out and reinserted. The reinsertional torque was measured in the same manner. Finally, the threads within the hole were destroyed using a "stripping screw." The screw was advanced and retracted through pushing and pulling (in a rasping motion). Adequate stripping was defined as less than 0.4 inch‐pounds of torque for reinsertion of the 35‐ × 6.5‐ mm screw into the failed hole. Stripping the hole was performed to simulate pedicle screw failure. This correlates with a severely failed or stripped pedicle screw.For each pedicle, an experimental screw was inserted into the stripped hole. The experimental screws varied in length, diameter, and presence of a shim. The diameter changed from 6.5 mm to 7.5 mm and 8.5 mm. The length changed from 35 mm to 40 mm and 45 mm. Shims were also added to some of the specimens. These combinations were varied throughout the vertebral levels T10 to L5. The randomization of the experimental design was achieved by using a modified Latin square.For each pedicle, the insertion torque for the standard control screw (35 × 6.5 mm) and the experimental screws were measured. The change in insertional torque, defined as the torque of the experimental screw minus the torque of the control screw, was the primary response variable. The effects of screw length and diameter were examined using analysis of variance for the Latin square design to examine an incomplete 3 × 3 factorial arrangement. The effect of using shims was examined using an analysis of variance comparing torque between paired pedicles using the same screw diameter and length.Sample size was based on samples in previous mechanical studies.5,11 Examining length and diameter without shim, controlling the probability of Type I error at alpha = 0.05, a sample of 8 pedicles for each combination of length and diameter would have a 95% power to detect a difference of 1 standard deviation between diameters, 95% power to detect a difference of 1 standard deviation, and 90% power to detect difference of 1 standard deviation between combinations of diameter and length which is considered the "interaction" effect. Therefore, 64 pedicles were required. An additional 32 pedicles were used for examining shims (32 pedicles with shims matched with 32 pedicles without shims). An additional 32 pedicles were available to replace rejected or failed pedicles and to examine differences between the combinations further. Therefore, 128 pedicles for 64 vertebrae from 8 spines were required for the study.ResultsThe bone mineral density of the specimens varied between 0.67 and 1.05 g/cm2. All the specimens were osteoporotic. Because of the design of the experiment, there were sufficient extra pedicles. Although some pedicle screw placements resulted in cortical breech, there were enough additional pedicles to compensate for these failures.Reinsertional TorqueRemoving the screw and simply replacing it caused a 34.1 ± 3% (SE) decrease (P < 0.000005) for the control screws and a 33.6 ± 3.8% decrease (P < 0.000005) for the test screws. These P values were obtained by t test for paired samples. The change in insertional torque for simply removing and replacing the initial screw was compared with the reinsertional torque of removing and reinserting the salvage screw. There was no difference (P = 0.91; t test for paired samples).Change in DiameterIncreasing the diameter of the revision screw by 2 mm resulted in a significant improvement in the insertional torque. Increasing the diameter by 1 mm caused the insertional torque to decrease by 68.8 ± 6.8% of the original. But increasing the diameter by 2 mm caused the insertional torque to increase by 8.4 ± 12.1% of the original (P = 0.004, Mann‐Whitney test).Change in LengthThe effect of only changing the length of the revision screw did not appear to cause a significant restoration of the insertional torque. Increasing the length by only 5 mm caused the insertional torque to decrease by 69.8 ± 4.3% of the original insertional torque, whereas increasing the length by 10 mm caused a 58 ± 9.2% decrease (P = 0.2118; Mann‐Whitney test).Changing Length and DiameterFor the extremes of diameter‐that is, the diameter change of 0 and 2 mm it did not appear that changing the length had a substantial effect. But with a diameter change of 1 mm, increasing the length of the screw appeared to cause a significant increase in the insertional torque (P = 0.0094, Kruskal‐Wallis; 1‐way analysis of variance). These values are listed in Table 2. This suggests that for the 7.5‐mm diameter screw (1‐mm increase), increasing the length of the screw caused a significant increase in the insertional torque. Thus, there was an interaction effect between these variables for the 7.5‐mm diameter screw.Table 2. Effect of Changing Length for 1.0‐mm Increase in DiameterUsing a ShimUsing a shim did not cause a substantial improvement in the insertional torque (P = 0.77; analysis of variance after the data was logarithmically transformed to satisfy assumption of normality and homogeneity). The comparison of the values obtained is listed in Table 3. In Figure 1, the trends of the data obtained and the compiled data are shown.Table 3. Effect of Using Shims for the Various Changes in Length and DiameterFigure 1. A screw with a 2‐mm increase in outer diameter engages the bone along the entire length of the screw.Torque With Bone Mineral DensityFor the specimens within this range of bone mineral density, there was not a significant correlation between the bone mineral density and the insertional torque (P = 0.332 and r = 0.18; Spearman correlation coefficients).Variation of 35‐ × 6.5‐mm Screw With Vertebral LevelThe values for the insertional torque for the control screws were analyzed. It was determined that there did not exist a significant correlation between the insertional torque and the vertebral level (P = 0.96; analysis of variance).Left to Right ComparisonLeft to right comparison of the insertional torque for the original control screw was also evaluated. On the left side, the mean torque was 4.3 ± 0.3 in.‐lb); on the right side, the mean torque was 4.3 ± 0.4 in.‐lb), (P = 0.21, analysis of variance). This would indicate consistency in screw placement and technique.DiscussionRepairing failed pedicle screws has become the topic of a number of investigations.11,14,25 Whether in‐line axial pullout or cephalocaudad toggle should be used as a failure model has been debated. Clinical experience suggests that if the screw does not break, there will be a "windshield wiper" effect surrounding the screw with resultant bone loss. Removal of a screw that has broken before toggling occurs often requires excavation of surrounding bone. Therefore, screw salvage or revision may have to make up for substantial bone loss. The technique used in the current study created a defect that correlates with a substantially failed pedicle screw. The reinsertional torque of less than 0.4 in.‐lb correlates with no palpable resistance placing the screw into the failed hole. This closely approximates the clinical situation encountered with a failed screw.The most mechanically successful method of pedicle screw salvage occurs with the application of PMMA.14,25 Pfeifer et al14 showed that using low‐pressure PMMA yielded a 149% increase in pullout strength. Zindrick et al25 used pressurized PMMA to salvage failed screws. They found that use of PMMA doubled the pullout strength. Although this method appears to be efficient, the possible side effects are not inconsequential. These side effects include possible extravasation through a fractured cortex, thermal damage, difficult removal in the event of infection or misplacement, and the potential for neurologic compromise. For this reason, the authors have chosen not to use PMMA in the clinical setting. Other newer fracture grouts such as calcium carbonate (Skeletal Reconstruction System; Norian Corp., Mountain View, CA) may offer significant advantages in comparison with PMMA.12Another alternative for hole augmentation is placing bone into the hole. Bone can be structural corticocancellous bone, often called matchsticks or shims, or nonstructural, milled bone. Examined only the matchstick bone alternative was examined. In the current study, placing shims did not result in a significant improvement in insertional torque after the hole was stripped. This result is supported by the results from the investigation by Pfeifer et al.14 They also stated that matchstick reconstruction was not recommended. They found that matchstick reconstruction provided the least improvement in their axial load testing: Placing matchstick bone within the hole yielded 56% of the original strength.13 There was a difference in results in specimens with low original insertional torque compared with those with high original insertional torque. Although they did not specifically measure bone mineral density, it is probable that the low insertional torque specimens were osteoporotic. The specimens with high insertional torque had better pullout testing with all of the revision techniques. Also, Halvorson et al9 found that packing a stripped pedicle screw hole with corticocancellous bone did not affect the strength of pedicle screw pullout in osteoporotic spines. Adding a shim does not increase thread engagement into the pedicle. Considering that pedicle fill is a dependent factor when trying to increase pedicle screw fixation strength, adding a matchstick shim does not effect the pedicle fill of the revision screw.2 Although it may not be biomechanically optimal, when faced with a difficult revision, the surgeon may still consider this as an adjunct.McLain et al11 investigated the effect of using larger diameter screws as a method of pedicle screw salvage. They investigated the strength of using a 7‐ and 8‐mm diameter screw as a salvage technique for a failed 6‐mm screw. This study did not vary the length of the salvage screw. They found that the 8‐mm diameter screw was stronger than the 7‐mm diameter screw. They also concluded that the 8‐mm VSP (variable screw placement) screw was stronger than the 8‐mm CD (Cotrel‐Dubousset) screw. In these studies, pedicle failure, before the application of salvage techniques, occurred through axial loading when the screw pulled out by the distance of one thread.Increasing the diameter of the salvage screw resulted in increased pedicle fill, allowing the threads to engage the intact bony outer edge, thus causing increased insertional torque (Figure 1). Increasing the length resulted in only the most distal threads' engaging new bone (Figure 2). This distal bone is weak cancellous bone, and it can easily fail. In the current study, none of the screws penetrated the anterior cortex, so it cannot be said how much strength this would add.Figure 2. A longer screw of the same outer diameter inserted into a stripped hole engages only the bone at the tip of the screw.The effect of increasing both the length and the diameter was only significant with the 7.5‐mm screw (1‐mm increase). In this situation, the 1‐mm increase in diameter may not optimize the screw‐bone interface. Therefore, adding length to this screw increases its insertional torque. Increasing the diameter by only 1‐mm may not optimize the pedicle fill. Brantley et al2 found that in nonosteoporotic bone, pedicle fill contributes to the fixation strength. Bernard et al1 found that choosing the largest diameter screw possible for each pedicle increases the resistance to pullout.A major observation made during this study was the loss of torque resulting from simply backing out a screw and replacing it. This action resulted in a 34% decrease in original torque‐a significant loss of strength. This result is important because of the desire to reposition a screw intraoperatively. During in vivo pedicle screw placement, it can be tempting simply to remove a screw and replace it in its original hole. The current results strongly discourage this action and show that maximum torque is obtained when the screw is initially placed. Because the pedicle screws are the strongest interface between longitudinal instrumentation and the bone, any compromise in this interface may decrease the rigidity of the construct and may decrease the fusion rate.20There was no correlation between the bone mineral density and the insertional torque. Many other investigators have found correlations between bone mineral density and axial pullout strength.3,7,9,13,15,19 A few others have also observed a correlation between bone mineral density and the insertional torque.3,13,15,17,24 The current results did not support this finding; however, the specimens were all osteoporotic and that may have been responsible for this finding. Brantley et al2 found that with osteoporotic specimens there is a low correlation between bone mineral density and the fixation stiffness itself.The effect of pedicle fill was also examined in the current study. The change in insertional torque was determined in relation to the level of insertion for the 35‐ × 6.5‐mm screw. In each specimen, the L5 pedicle was larger than the T10 pedicle. For this reason, the same screw should have resulted in more pedicle fill because it was placed at higher levels. The results did not show a correlation between the vertebral level and the insertional torque. This result may be explained by the size of the 35‐ × 6.5‐mm screw. Because it is small it did not truly approach optimal pedicle fill, even at the higher vertebral levels.There was, however, a left‐to‐right correlation. This supports the validity of the current data and the experimental design and the consistency of the screw insertion technique. It correlates well with the finding of many other investigators that biologic variability is minimized by making right‐left comparisons on the same specimen.Critical AssessmentSome may argue that using torque, not axial pullout, as a gauge of screw strength is not a true representation of the screw‐bone interface. There is correlation between axial strength and torque, as reported by several investigators.3,5,13,15,17,18,24 In addition to the mechanical support, measuring torque may be of greater benefit because it is the torque that surgeons appreciate when they place pedicle screws in a patient.Another shortcoming of this experiment is the low range of bone mineral density measured in the samples. The behavior of pedicle screws may be different in osteoporotic versus nonosteoporotic bone.2 This factor may need consideration when interpreting these results. Halvorson et al9 report that when using pedicle screw fixation in osteoporotic spines, augmentation by offset hooks may be necessary. Simply having an osteoporotic spine does not preclude use of pedicle screws, but it may be that screws with an insertional torque of less than 4 in.‐lb should not be used.24Clinical ApplicationsThe conclusions drawn from the results of this experiment have useful clinical application. The most significant observation, the substantial loss of insertional torque when simply repositioning a pedicle screw, should be carefully considered by the surgeon who is tempted to back out and reinsert a screw. For pedicle screw salvage, the results suggests it is best to place a larger diameter screw. Preferably, it should be 2 mm larger in diameter than the original. If a 2‐mm increase in screw diameter is not tolerated by the pedicle dimensions, then placement of a screw 1‐mm larger in diameter that is longer (a 5‐ or 10‐mm increase) than the original, is necessary to restore optimal mechanical fixation. Finally, adding a shim does not significantly affect the salvage reinsertional torque in osteoporotic spines.ConclusionsRemoving and replacing an original screw results in a decrease in the insertional torque. Pedicle salvage is most effective when using a 2‐mm larger diameter screw. If a screw that is 2 mm larger in diameter will not fit, then using a 1‐mm larger and 5‐ to 10‐mm longer screw is the next best option. Using a matchstick shim in osteoporotic bone has little to no effect on the insertional torque for pedicle screw salvage.References1. Bernard TN, Seibert CE. Pedicle diameter determined by computed tomography- its relevance to pedicle screw fixation in the lumbar spine. Spine 1992;(Suppl)17:S160-3. [Context Link]2. 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Vaccaro AR, Rizzolo SJ, Allardyce TJ, Ramsey M, Salvo J, Balderston RA, Cotler JM. Placement of pedicle screws in the thoracic spine. Part 1: Morphometric analysis of the thoracic vertebrae. J Bone Joint Surg [Am] 1995;77:1193-9. [Full Text] [Medline Link] [Context Link]22. Vaccaro AR, Rizzolo SJ, Balderston RA, Allardyce TJ, Garfin SR, Dolinskas C, An HS. Placement of pedicle screws in the thoracic spine. Part 2: An anatomical and radiographic assessment. J Bone Joint Surg [Am] 1995;77:1200-6. [Full Text] [Medline Link] [Context Link]23. Zdeblick TA. A prospective, randomized study of lumbar fusion: Preliminary results. Spine 1993;18:983-91. [CrossRef] [Full Text] [Medline Link] [Context Link]24. Zdeblick TA, Kunz DN, Cooke ME, Mc Cabe R. Pedicle screw pullout strength: correlation with insertional torque. Spine 1993;18:1673-6. [CrossRef] [Full Text] [Medline Link] [Context Link]25. Zindrick MR, Wiltse LL, Widell EH, et al. 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[CrossRef] [Full Text] [Medline Link] [Context Link] bone mineral density; insertional torque; pedicle screw salvageovid.com:/bib/ovftdb/00007632-199806150-0001500007632_1994_19_1752_brantley_effects_|00007632-199806150-00015#xpointer(id(R2-15))|11065213||ovftdb|00007632-199408000-00016SL00007632199419175211065213P74[CrossRef]10.1097%2F00007632-199408000-00016ovid.com:/bib/ovftdb/00007632-199806150-0001500007632_1994_19_1752_brantley_effects_|00007632-199806150-00015#xpointer(id(R2-15))|11065404||ovftdb|00007632-199408000-00016SL00007632199419175211065404P74[Full Text]00007632-199408000-00016ovid.com:/bib/ovftdb/00007632-199806150-0001500007632_1994_19_1752_brantley_effects_|00007632-199806150-00015#xpointer(id(R2-15))|11065405||ovftdb|00007632-199408000-00016SL00007632199419175211065405P74[Medline 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Jr MD, LTC, MC; Orchowski, Joseph R. MD, CPT, MC; Ellenbogen, Richard G. MDBiomechanics1223