Clinical Results of the DSS System (semi-rigid stabilizer) for Lumbar Fusion. Pettine K, Siman H, Techy F

Transcription

1 Abstract Clinical Results of the DSS System (semi-rigid stabilizer) for Lumbar Fusion Pettine K, Siman H, Techy F Objective: Analysis of the DSS system to treat low back pain and stenosis with instability. Introduction: The optimal rigidity of a pedicle screw based implant for achieving lumbar fusion has not been established. Stability is clearly needed for fusion, however, too much of it may interfere with new bone formation and be detrimental to the adjacent levels. Methods: This is a prospective series of lumbar fusion cases utilizing the DSS, from August 2008 through August N=239 consecutive patients. The DSS is a semi-rigid pedicle screw based device. Groups were divided based: on diagnosis (discogenic back pain vs stenosis with instability), rigidity of the implant used (semi-rigid vs more rigid, with or without interbody device), number of levels (1, 2, or multiple). Pre and post operative Oswestry disability index (ODI) and Pain Visual Analog Scores (VAS) forms were collected and compared among groups. Results Overall, 50% of patients had greater than 15-point improvement in ODI at two-year follow-up. There was a clear difference in the clinical results among the groups. In summary, results ranged from only 33% of patients having more than 15 point improvement in the group of: 2 level, semi rigid fixation and no interbody spacers to 70% patients with more than 15 point improvement in the group: 2 level, semi rigid fixation, with interbody devices. The addition of interbody devices to the construct seemed to have improved the percentage of 15 point improvement on ODI from less than 40% to 60% of patients. Out of the 239 patients, only 3 needed further surgery to treat symptomatic adjacent level disease. (Based on historical literature predictions, this series would have 11 patients that would need surgery for this reason if rigid implants were used). Conclusion: The DSS seems to be adequate to stabilize the spine to treat stenosis with instability or discogenic back pain. Overall results are similar to those in the literature of rigidly instrumented spines. Adding stability by adding an interbody spacers, dramatically improved the clinical results and were considered advisable by the authors for some groups where more stability was needed, especialky when treating back pain with or without radiculopathy. The need for surgery at adjacent segments was smaller for the DSS group when compared to the rigid instrumentation literature. 1

2 Introduction According to the National Center for Health Statistics, low back pain affects approximately 90% of the US population at some time in their lives. 15 Chronic severe low back pain lasting more than six months is often due to age-related intervertebral disc degeneration, traumatic annular tears, facet joint arthrosis or central, lateral recess and foraminal stenosis with or without segmental instability. 1 A percentage of these patients may be appropriate surgical candidates. 41 Lumbar fusion may be appropriate depending upon the severity of the patient s pre-operative clinical condition and lack of surgical risk factors for failure such as drug addiction, alcoholism, lack of social support, history of psychological issues. 2 Lumbar fusion has been well accepted as an appropriate treatment for infectious conditions of the spine, progressive spinal deformities and traumatic injuries. 40 Lumbar fusion has become an accepted surgical technique in patients with 3,4, 9, 43 demonstrated lumbar instability such as degenerative spondylolisthesis. Some authors as Farfan, Kirkaldy-Willis, Paris, Frymoyer, Panjabi and Hanley will define lumbar segment instability not only as motion detected on dynamic or standing radiographs, but also as the spine becomes unable to tolerated physiological external loads, resulting in pain, or placing neurological structures at risk. 5-8 Nevertheless, most studies that correlate fusion rates or clinical results with stabilization of the segment, use the detected motion or deformity parameter to define instability. The utilization of pedicle screws has increased reported fusion rates from 65% without instrumentation to up to 95% or greater with pedicle screw instrumentation The clinical success rates following lumbar fusion are generally reported to be between 50-70%. 9 So, while obtaining a solid fusion does not guarantee good clinical outcome, patients that develop nonunions seem to do worse than their counterparts with a solid fusion. Another question that arises not only in spine fusion surgery but also in long bone fracture healing is how much stability is optimal to promote bone growth and union, as we are all very familiar with the concept that controlled micromotion and weight bear do stimulate bone growth. Another point to take into consideration is that one of the accepted inherent complication of rigid lumbar fusion is the development of adjacent level degeneration In an attempt to improve the problems related to 1) obtaining optimal stability for fusion and 2) preventing adjacent segment degeneration, many semi-rigid implants were created to stabilize the cervical, thoracic, lumbar spine, as well as long bones. This paper will evaluate the clinical performance of one of these devices: the pedicle screw based DSS stabilization system (Paradigm Spine, New York, NY) This paper is a prospective series of lumbar fusion cases utilizing the DSS, from August 2008 through August N=239 consecutive patients. The DSS can be used as a semi-rigid pedicle screw based device as well as a rigid system based on the connector used between pedicle screws. Groups were divided based: on diagnosis (discogenic back pain vs stenosis with instability), rigidity of the implant used (semi-rigid vs more rigid, with or without interbody device), number of levels (1, 2, or multiple). Pre and post operative Oswestry disability index (ODI) and Pain Visual Analog Scores (VAS) forms were collected pre and post operatively and compared among groups. Materials and Methods 2

3 This paper represents a prospective non-randomized research of every surgical procedure performed utilizing the DSS implant by the senior author from August 2008 through There were a total of 239 patients. Each year represents a progression of clinical experience as well as implant availability and improvement. In 2008, the rigid coupler was not available. All cases in 2008 were done utilizing only the slotted coupler without an interbody fusion. Two thousand nine represents the utilization of a composite construct consisting of a solid coupler with interbody fusion and the slotted coupler without interbody fusion and 2011 represent utilization of an interbody fusion with both the rigid and slotted couplers except at the most rostral level. Wilke, et al. has published on the history of the development of the DSS stabilization system in an article entitled Prospective Design Delineation and Subsequent In-vitro Evaluation of a New Posterior Stabilization System. 16 The FDA off-label use for the DSS system is included in the package inserts. FDA indications for use of the semi-rigid coupler include single- or multi-level pedicle screw lumbar fusion for a diagnosis of degenerative spondylolisthesis, kyphosis and failed previous fusion. The rigid and slotted couplers are not FDA approved to be used together. The rigid coupler is FDA approved for a single-level pedicle screw implant with fusion for degenerative disc disease, spondylolisthesis, trauma, spinal stenosis, curvature, tumor and pseudoarthrosis. All other uses of these implants are FDA off-label. Every patient in this study was evaluated with pre-operative physical examination including an oswestry disability index (ODI); visual analog scale (VAS); AP, lateral and when indicated flexion/extension radiographs and MRI scanning. When clinically indicated, discography was performed to ascertain symptomatic discogenic annular tears. Follow-up was obtained at six weeks, three months, six months, one year and two years. Follow-up included physical examination, AP and lateral radiographs, ODI and VAS. This is a prospective non-randomized study. Data stratified by groups: Group 1 Surgeries performed in Dynamic fixation (slotted coupler) without interbody spacers: The least stable of the scenarios analyzed. The least favorable clinical results. Early surgical experience. All procedures in 2008 were performed with slotted couplers and posterolateral fusion utilizing locally harvested morcellized autograph augmented with DBM. There were no interbody implants or fusions. In 2008, 15 patients had undergone a previous rigid pedicle screw and rod lumbar fusion and developed adjacent level degeneration above their fusion and associated facet hypertrophy creating lateral recess and foraminal stenosis. Ten patients had a diagnosis of discogenic low back pain secondary to an annular tear (artificial disc candidates) and eight patients had a primary diagnosis of stenosis with lumbar instability without previous surgery. In 3

4 2008, the surgical procedure in the 15 patients with previous fusion and adjacent level degeneration consisted of removing the previous implants and extending the fusion with the slotted coupler. The 10 patients with discogenic pain who were candidates for an artificial disc replacement underwent placement of a slotted coupler at either one or two levels. All of the patients with a pure diagnosis of stenosis with instability without previous surgery underwent decompression and stabilization with a slotted coupler Surgeries n=33 21 Male, 12 Female Avg. Age: 50.8 years Avg. BMI: 31.0 Avg. EBL: cc Avg. Op. Time: min. No rigid coupler available Procedure Performed # of Patients Hardware removal w/ DSS 15 - adjacent level degeneration 8 - stenosis with instability DSS only (no previous surgery) 10 - discogenic low back pain (artificial disc candidates) Re-ops/Revisions: Two 2-level patients were revised to rigid fusion at 10 months and 12 months after their index slotted coupler fusion with diagnosis of possibly ongoing discogenic low back pain secondary to an annular tear. 1-Level: 15 (all slotted couplers) 2-Level: 17 (all slotted couplers) 3-Level: 1 (slotted coupler) Total 2008 DSS ODI and VAS (n=33) ODI 49.8 (n=33) 33.8 (n=32) 33.3% Avg (n=26) p= % Avg. VAS (mm) 77.9 (n=33) (n=32) 34.6% >15 pt. ODI at 2 years (n=23) Level ODI and VAS (n=15) ODI 48.1 (n=15) 33.1 (n=15) 39.0 (n=14) p=0.0639

5 30.6% Avg. VAS (mm) 75.0 (n=15) 36.0 (n=15) 28.4% Avg (n=12) 38.5% >15 pt. ODI at 2 years Level ODI and VAS (n=17) ODI 51.3 (n=17) 34.8 (n=16) 37.8% Avg (n=11) p= % Avg. VAS (mm) 80.3 (n=17) 40.5 (n=16) 45.5% >15 pt. ODI at 2 years (n=10) Group 2: Cases performed in Patients in this group had a dynamic fixation (bilateral slotted couplers and no interbody fusion) if surgery was performed at 1 level. Patients had a hybrid construct (slotted couple on one side and rigid connector on the contralateral side plus interbody fusion if the surgery required more levels.) The exception was if the indication of surgery was for back pain only, bilateral slotted connectors and no interbody fusion were used regardless of number of levels fused. In this group, a total of 95 patients underwent a DSS implant surgery. The pre-operative diagnosis consisted of discogenic low back pain secondary to an annular tear in 11 patients, all of which were candidates for an artificial disc replacement. Eighty-four patients had a diagnosis of stenosis, either secondary to a previous fusion and development of adjacent level degeneration or stenosis with instability without prior surgery. Thirty-nine patients had removal of their previous instrumentation and extension of the fusion secondary to adjacent level degeneration associated stenosis. Forty-five patients had placement of a slotted coupler if it was a single-level case or a composite procedure consisting of a rigid and slotted coupler if it was two or more levels. The 11 patients with discogenic low back pain underwent only a slotted coupler either at one or two levels. In 2008 and 2009, 21 patients underwent a one- or two-level fusion with a slotted coupler with discogenic low back pain (artificial disc candidates). Six of these patients had minimal improvement in ODI or VAS (five males, one female). All of these patients had a diagnosis of discogenic low back pain secondary to an annular tear. Two were revised to a rigid fusion. Based on these results, patients with this diagnosis are now treated with an interbody cage and slotted coupler. Rigid coupler available Procedure Performed 5 # of Patients

6 2009 Surgeries n=95 49 Male, 46 Female Avg. Age: 53.3 years Avg. BMI: 29.1 Avg. EBL: cc Avg. Op. Time: min. Composite rigid and slotted coupler 52 Hardware removal slotted coupler 27 Slotted coupler only (no previous fusion) 16 I-Level: 23 (all slotted couplers) 2-Level couplers: 19 (all slotted couplers) Multi-Level couplers: 1 (all slotted couplers) 2-Level Composite: 21 Multi-Level Composite: 31 Re-Ops/Revisions/Complications: 1) 2-Level Composite patient had a dural tear. 2) Multi-Level Composite patient fractured a left L3 pedicle screw within three months post op which was revised. (male BMI 36) This large male fell and acutely fractured the pedicle screw. Eight mm screws were not available at the time but would now be used. 3) One fractured coupler was replaced with a new coupler 14 months post op. (male BMI 40) 4) One patient had an extension of their previous DSS fusion due to adjacent level degeneration. (index surgery in 2008) Be aware of potential hardware complications in patients with BMIs over 35. There was one fractured screw and one fractured coupler in two large males whose BMIs were 36 and 40. Neither of these patients had an interbody PEEK cage. Using anterior column support may be advisable in large patients when the slotted coupler is used. Total 2009 ODI and VAS ODI 58.6 (n=86) 34.4 (n=66) 40.5% Avg. VAS (mm) 80.2 (n=84) 37.8 (n=64) 61% >15 pt. ODI at 2 years (n=45) 43.9% Avg (n=41)

7 All procedures with only slotted couplers (one-, two- or multi-level) were performed with a posterolateral fusion utilizing locally harvested morcellized autograph augmented with DBM. The surgeries that were composites (slotted and rigid couplers) were performed with a complete laminectomy, facetectomy, TLIF and PEEK interbody fusion with a rigid coupler and only a posterolateral fusion at the levels with a slotted coupler Level Couplers ODI and VAS ODI 60.1 (n=23) 39.3 (n=12) 43.1% Avg (n=4) p= % Avg. VAS (mm) 78.9 (n=23) 49.0 (n=14) 40.0 (n=3) p= % >15 pt. ODI improvement at 2 years Level Couplers ODI and VAS ODI 55.5 (n=19) 39.3 (n=12) 52.1% Avg (n=6) p= % Avg. VAS (mm) 83.5 (n=19) 59.0 (n=10) 50.8 (n=6) 33.3% >15 pt. ODI improvement at 2 years Level Composite and interbody fusion ODI and VAS ODI 57.2 (n=21) 35.2 (n=16) 27.4% Avg (n=11) 41.0% Avg. VAS (mm) 81.5 (n=21) 35.9 (n=16) 29.1 (n=11) 63.6% 15 pt. ODI at 2 years Multi-Level Composite and interbody fusion ODI and VAS ODI 59.9 (n=31) 32.7 (n=26) 44.2% Avg (n=15) 38.0% Avg. VAS (mm) 79.0 (n=31) 34.2 (n=26) (n=12)

8 60.0% >15 pt. ODI improvement at 2 years. The patients that were submitted to the more rigid construct ( dynamic coupler and contralateral rigid rod plus interbody cage) did significantly better despite having more levels of surgery. Group 3: In 2010, there were 69 total patients underwent surgery with the DSS implant system. Dynamic and hybrid posterior fixation was available. Interbody cages were used in most cases. (Only 6 cases in 2010 and 2011 did not include interbody cages). Eight patients had a diagnosis of discogenic back pain with an annular tear (artificial disc replacement candidates) and 61 patients had a primary diagnosis of stenosis due to adjacent level degeneration from a previous fusion or central, lateral recess and foraminal stenosis with instability Surgeries n=69 35 Male, 34 Female Avg. Age: 53.0 years Avg. BMI: 29.9 Avg. EBL: cc Avg. Op. Time: min. Rigid coupler available Procedure Performed # of Patients Hardware removal w/ DSS 12 DSS w/ rigid coupler 19 DSS only (no previous fusion) 38 1-Level: 23 (all slotted couplers) 2-Level couplers: 24 (all slotted couplers) Multi-Level coupler: 3 (all slotted couplers) 2-Level composite: 12 Multi-Level composite: 7 Re-Ops/Revisions/Complications: Two patients had an extension of their previous fusion due to adjacent level degeneration (index surgery in 2008). A 2-level coupler patient had dural tear and a multi-level composite patient had dural tear. Total 2010 ODI and VAS Pre-Op 1 Year ODI 55.4 (n=69) 38.5 (n=55) 8

9 38.6% Avg. VAS (mm) 72.2 (n=69) 45.5 (n=54) 65.5% >15 pt. ODI improvement at 1 year Level Couplers ODI and VAS Pre-Op 1 Year ODI 56.4 (n=23) 40.4 (n=22) p= % Avg. VAS (mm) 74.1 (n=23) 45.2 (n=21) p= % >15 pt. ODI at 1 year. All but six single-level surgeries performed in 2010 and 2011 included an interbody PEEK cage placed either through a PLIF or TLIF approach. All multi-level cases had an interbody fusion at all but the most rostral level. All levels had a posterolateral fusion with locally harvested morcellized bone augmented with DBM Level Couplers ODI and VAS Pre-Op 1 Year ODI 56.3 (n=24) 37.6 (n=18) 52.1% Avg. VAS (mm) 72.1 (n=24) 51.8 (n=18) p= % >15 pt. ODI at 1 year Level Composite Pre-Op 1 Year ODI 53.1 (n=12) 44.9 (n=7) p= % Avg. VAS (mm) 68.5 (n=12) 43.3 (n=7) p= % >15 pt. ODI at 1 year Multi-Level Coupler Pre-Op 1 Year ODI 42.3 (n=3) 24.0 (n=2) 9

10 p= % Avg. VAS (mm) 75.0 (n=3) 30.0 (n=2) (p=0.0696) 100% >15 pt. ODI improvement at 1 year Multi-Level Composite Pre-Op 1 Year ODI 59.1 (n=7) 31.3 (n=6) p= % Avg. VAS (mm) 70.0 (n=7) 35.8 (n=6) p= % >15 pt. ODI at 1 year. The addition of interbody cages seem to have improved the clinical results. Group 4: All surgeries, with the exception of one, were done utilizing interbody spacers. In 2011, there were 42 total patients. Fourteen procedures involved previous instrumentation and extension of the fusion level. A total of 23 patients had only slotted couplers and DSS with no previous fusion and five cases with composite procedures. Rigid coupler available Procedure Performed # of Patients One-level coupler w/o interbody cage 1 One-level coupler w/ interbody cages 20 Two-level couplers w/ interbody cages 6 Three-level couplers w/ interbody cages 1 Composite rigid and slotted couplers w/ interbody 10 cages Three-level composite couplers w/ interbody cages Surgeries N=42 19 Male, 23 Female Smokers: 7 (35 non-smokers) Average Age: 56.3 years Average BMI: 31.9 Average EBL: cc Average Op Time: min 1-Level Couplers: 21 (all slotted couplers) 2-Level Couplers: 6 (all slotted couplers) 10

11 Multi-Level Couplers: 1 (all slotted couplers) 2-Level Composite: 10 Multi-Level Composite: 4 Complications/Revisions: 1 patient had a deep surgical wound infection treated with a wound vac. Total 2011 ODI and VAS Pre-Op 3 Month 6 Month 33.1 (n=22) 35.2% Avg. ODI 51.1 (n=42) 34.9 (n=26) 31.7% Avg. VAS (mm) 74.0 (n=42) 37.0 (n=25) 44.0 (n=21) Level Coupler ODI and VAS Pre-Op 3 Month 6 Month ODI 49.8 (n=22) 32.5 (n=12) p= % Avg (n=13) p= % Avg. VAS (mm) 75.9 (n=22) 36.3 (n=12) p= (n=13) p= Level Coupler ODI and VAS Pre-Op 3 Month 6 Month ODI 50.9 (n=9) 32.0 (n=6) p= % Avg (n=5) p= % Avg. VAS (mm) 75.5 (n=9) 36.3 (n=6) p= (n=5) p= Level Composite ODI and VAS Pre-Op 3 Month 6 Month ODI 58.1 (n=6) 42.7 (n=3) p= % Avg (n=3) p= % Avg. VAS (mm) 68.0 (n=6) 46.7 (n=3) p= (n=3) p= Note: Not enough data for Multi-Level Couplers and Multi-Level Composite surgical patients. 11

12 Total DSS ( ): N=239 1-Level Slotted Couplers: 83 2-Level Slotted Couplers: 69 Multi-Level Slotted Couplers: 7 2-Level Composite: 39 Multi-Level Composite: 41 Average Operative Times for all 239 surgeries: One-level: 105 minutes Two-level: 154 minutes Three-level: 235 minutes A total of 197 patients underwent surgery between 2008 and 2010 with the DSS implant system. This group was analyzed separately to allow a minimum one-year follow-up. DSS Combined ODI 56.0 (n=197) 35.6 (n=155) 36.4% Avg. VAS (mm) 77.0 (n=197) 40.3 (n=152) 47.8% Avg. 49.2% >15 pt. ODI at 2 years (n=71) 41.3% Avg (n=64) 53.6% Avg. Actual In Window 2 year follow-up: 61% NOTE: 15 patients gave 2 year follow-up data over the phone and were added. 2 patients were found to be deceased. Couplers vs. Composite Comparison Couplers: ODI and VAS (n=126) ODI 52.5 (n=126) 36.8 (n=80) () 37.0% Avg (n=36) () 36.8% Avg. VAS (mm) 76.7 (n=126) 57.2 (n=77) p< (n=32)

13 Composites: ODI and VAS (n=71) ODI 57.8 (n=71) 34.8 (n=55) () 36.1% Avg (n=26) () 38.3% Avg. VAS (mm) 77.3 (n=71) 36.0 (n=55) 1-Year ODI: p= Year VAS: Year ODI: p= Year VAS: (n=23) There is no statistical difference between couplers and composite ODI and VAS. Patient Diagnosis by Year Discogenic low back pain Stenosis Adjacent level degeneration Total Patients through 2010 Lumbar Surgeries by Primary Diagnosis Discogenic low back pain: 29 patients Stenosis (including spondylolisthesis and instability): 114 patients Adjacent level degeneration above a previous fusion: 54 patients Patient Demographics Discogenic LBP Stenosis Adjacent level degeneration Total Patients Average Age (yrs) Smokers 14 (25.0%) 23 (26.4%) 5 (22.7%) 42 (25.1%) Average BMI

14 Oswestry Disability Index Discogenic LBP Stenosis Adjacent level degeneration Pre-op Range Comparison p-value Month Range p-value 1 <0.001 <0.001 <0.10 Comparison p-value Month Range p-value 1 <0.001 <0.001 <0.071 Comparison p-value Month Range p-value 1 <0.001 <0.001 <0.048 Comparison p-value p-value obtained using two-sample t-test 2 p-value obtained using the one-way ANOVA test; alpha = 0.05 Based on the ANOVA rest results, we cannot reject the null hypothesis that the means are the same. Therefore, we must assume that the pre-op and post-op ODI averages are not significantly different. However, both the discogenic LBP and stenosis patients significantly improved post-operatively while the adjacent level patients barely did at 24 months post-op. Visual Analogue Scale Discogenic LBP Stenosis Adjacent level degeneration Pre-op (mm) Range Comparison p-value Month (mm) Range p-value 1 < < < Comparison p-value Month (mm) Range p-value 1 < < < Comparison p-value Month (mm) Range p-value 1 < < < Comparison p-value p-value obtained using two-sample t-test 2 p-value obtained using the one-way ANOVA test; alpha = 0.05 All three groups showed significant improvement in VAS scores post-operatively; again, however, the discogenic LBP patients showed the most improvement while the adjacent level patients showed the least. Male vs. Female Comparison 14

15 Females: Males: n=92 n=105 Avg. Age: 53.4 years Avg. Age: 52.2 years Avg. BMI: 29.2 Avg. BMI: 30.0 Avg. EBL: cc Avg. EBL: cc Avg. Op. Time: min. Avg. Op. Time: min. Females: ODI and VAS ODI 51.3 (n=92) 40.2 (n=63) () 44.1% Avg. VAS (mm) 68.9 (n=92) 40.1 (n=62) 43.3% >15 pt. ODI at 2 years (n=30) () 33.6% Avg (n=27) Males: ODI and VAS ODI 50.9 (n=105) 34.3 (n=68) () 31.2% Avg. VAS (mm) 69.0 (n=105) 39.3 (n=69) 49.7% >15 pt. ODI at 2 years (n=38) () 33.2% Avg (n=37) 1-Year ODI: p= Year ODI: p= Year VAS: p= Year VAS: p= There is no statistical difference between male and female surgical patients. BMI <30 vs. BMI >30 Comparison BMI <30: BMI >30: n=94 n=103 Avg. Age: 53.1 years Avg. Age: 50.5 years Avg. EBL: cc Avg. EBL: cc Avg. Op. Time: min. Avg. Op. Time: min. BMI <30 ODI and VAS 15

16 ODI 45.6 (n=94) 33.6 (n=66) (p<0.01) 26.4% Avg. VAS (mm) 63.4 (n=94) 46.3 (n=66) BMI >30 ODI and VAS 45.8% >15 pt. ODI at 2 years (n=24) (p<0.01) 27.1% Avg (n=23) ODI 51.9 (n=103) 36.7 (n=70) () 29.0% Avg. VAS (mm) 70.6 (n=103) 40.2 (n=70) 47.2% >15 pt. ODI at 2 years (n=36) () 32.1% Avg (n=32) 1-Year ODI: p= Year ODI: p= Year VAS: p= Year VAS: p= There is no statistical difference in ODI and VAS between BMI<30 and BMI >30. Between August 2008 and December 2011, a total of 239 patients underwent surgery with the DSS implant. Eighty-three patients had a one-level slotted coupler, 69 patients had a two-level slotted coupler, seven patients had more than two levels of slotted couplers, 39 patients had a two-level composite and 41 patients had a multi-level composite lumbar fusion. The results indicate a statistically significant improvement in ODI and VAS from pre-operative to one-year and two-level follow-ups to a p-value of < This series also represents a development in surgical technique on a year-to-year basis. Only the slotted coupler was available for all surgeries performed in The demographics of this group indicate a younger age (compared to 2009, 2010 and 2011) of 51 years. Ten of the 33 surgeries (close to 30%) in 2008 were performed with a pre-operative diagnosis of discogenic low back pain as opposed to the vast majority of patients in subsequent years who had a diagnosis of spinal stenosis with instability. The 2008 patients showed a statistically significant improvement in ODI and VAS to a p-value of < at both one- and two-year follow-up. However, only 34.6% of these patients had more than a 15-point improvement in ODI at two years. The average patient improved 28% at two-year follow-up. Two patients with a diagnosis of discogenic low back pain secondary to an annular tear were revised within a year with a rigid coupler and interbody fusion. They had a second surgery because of ongoing severe discogenic pain post op. It was felt the slotted coupler did not adequately decrease the compressive forces 16

17 across the disc space resulting in a lack of significant clinical efficacy. Four additional patients in the 2008 and 2009 groups had minimal improvement in ODI or VAS due to similar circumstances. All six patients had a diagnosis of discogenic low back pain secondary to an annular tear. Following this experience, patients with this diagnosis are no longer treated with a slotted coupler without an interbody implant. The surgeries performed in 2009 had the availability of both the rigid and slotted couplers. A total of 95 patients were operated on in This overall group had a statistically significant improvement in ODI and VAS at two-year follow-up to a p-value of < However, 53.3% of these patients had a 15-point improvement in ODI at two years and the average patient improved 33% at two-year follow-up, which were better clinical results than from the 2008 patients. The predominant pre-operative diagnosis in these patients was central, lateral recess and foraminal stenosis with instability as opposed to the 2008 group which in 10 of 33 patients was discogenic low back pain. There were 69 patients operated on in As in 2009, the predominant diagnosis was central, lateral recess and foraminal stenosis with instability. All but six single-level surgeries performed in 2010 (5) and 2011 (1) included an interbody PEEK cage placed either through a PLIF or TLIF approach. Multi-level surgeries had an interbody implant at all but the most rostral level. Again, there was a statistically significant improvement in ODI and VAS at one-year follow-up with the average patient improving 38.6% (ODI), and 65.6% of the patients had greater than a 15-point improvement in ODI and VAS at one-year follow-up. These were better results than 2009 and especially The 2008 through 2010 follow-up results based on pre-op diagnosis were interesting. These years were chosen to allow minimum one-year follow-up. There were 29 patients with a diagnosis of discogenic low back pain, 114 patients had a diagnosis of stenosis with instability and 54 patients had adjacent level degeneration above a previous fusion. Patients with a diagnosis of discogenic low back pain and stenosis had a very significant improvement in ODI and VAS () with no statistical difference between the groups. Patients with adjacent level degeneration had very statistical improvement in VAS () but barely significant improvement in ODI (p<0.048). These results emphasize that long-term impairments (as represented by the oswestry disability index) after previous lumbar fusion are often minimally improved with a second fusion procedure. Pain is improved (VAS) but not impairment (ODI). Sub-groups were broken out to evaluate the clinical results of males versus females and patients with a BMI less than 30 versus more than 30. A total of 92 females and 105 males with two-year follow-up were compared. Both groups demonstrated a statistically significant improvement in ODI and VAS at one- and two-year follow-up. There was no statistical difference in the clinical results between males and females. There were 94 patients with a BMI less than 30 and 103 patients with a BMI over 30 available for two-year follow-up. Both groups demonstrated a statistically significant improvement in ODI and VAS () with no statistical difference in this improvement in patients with less than versus more than a BMI of

18 Patients with a diagnosis of stenosis and instability clinically did better than those with a diagnosis of degenerative disc disease or discogenic low back pain secondary to an annular tear unless an interbody implant was included in the procedure. The percentage of discogenic low back pain patients with greater than a 15-point improvement in ODI at two years in the group with one-level slotted couplers without an interbody implant was 38% and 45% with two-level couplers. The clinical results improved dramatically when an interbody implant was utilized as demonstrated in 2009 when greater than 60% of patients with a composite construct had greater than 15-point improvement in ODI at two-year follow-up compared to the patients without an interbody implant and a two-level slotted coupler where only 33% of patients had greater than 15-point improvement in ODI. In 2010 and 2011, when most patients had an interbody fusion, greater than 60% had more than 15-point improvement in their ODI which even increased to greater than 70% of patients with a two-level slotted coupler and interbody fusion having greater than 15-point improvement in ODI at one-year follow-up. Discogenic low back pain patients can be successfully treated with the slotted coupler if an interbody implant is placed. The annulus in some of these patients appears to be ultrasensitive, which may be the etiology of their severe chronic discogenic back pain. The slotted coupler by itself does not appear to prevent compressive forces across the disc and thus does not provide adequate pain relief in about 65% of these patients. Discussion This paper represents a prospective non-randomized study of every patient operated on by the senior author utilizing the DSS stabilization system for lumbar fusion. The DSS stabilization system is unique in allowing the surgeon to utilize either a rigid interpeduncular connector versus the less rigid slotted connector. The slotted connector allows the interpeduncular distance to increase between 1.6mm to 2.6mm in flexion depending on the starting length of the connector. The slotted connector reduces overall range of motion at a specific spinal motion segment by 30-50%. 16 The DSS stabilization system for lumbar fusion allows the surgeon to incorporate the concepts articulated by McAfee. 14 That not all spinal fusions are biomechanically equal. Spinal arthrodesis is not an all or none phenomena. Spinal fusions are not biomechanically equal. A hypertrophic fibrous union can be more stable than a spinal segment with bony continuity if the cross-sectional area of the contiguous bone mass is small. McAfee references John Charnley s article on the closed treatment of tibial fractures. 52 Charnley recognized the fact that a hypertrophic non-union of the tibia is biomechanically stronger compared to an atrophic bony union. The first biomechanical study published by McAfee emphasizes this point with anterior lumbar fusions in a canine model. 13 The point that all spine fusions are not created equal is also emphasized in a symposium published in the Spine Journal. 39 The authors of this symposium all opined a different definition of what constitutes a successful spinal fusion. The point is different clinical situations in the spine may be optimally treated by different degrees of fusion rigidity. It is McAfee s contention that with different goals of spinal stability and gradations of spinal mobility, the flexibility of the posterior fusion mass should be different, and the modulus of elasticity of the spinal instrumentation should be offered in a range. Bone graft should be used to 18

19 allow the biology of a posterolateral fusion mass to match the biomechanical characteristics of the spinal implants. 14 This concept of residual sagittal motion after lumbar fusion has also been explained with a finite element analysis by Bono, et al. 53 Perhaps not all lumbar spine pathology is optimally surgically treated with the exact same amount of fusion rigidity. To maximize clinical results, perhaps the flexibility of a lumbar fusion mass should be different and the modulus of elasticity of the spinal instrumentation should reflect this. This concept may be one reason why the clinical results of rigid lumbar fusion range from 50-70% while the fusion rates are over 95% Panjabi 6,8, describes the whole concept of the NEUTRAL ZONE. The neutral zone is the initial portion of segmental range of motion during which spinal motion is produced against minimal internal resistance. The elastic portion of range of motion is the portion nearer to the end-range of movement that is produced against substantial internal resistance. In vitro studies by Panjabi in 1992 and 1994 as well as Oxland with Panjabi in 1992 suggest an increase in the size of the neutral zone may be a better indicator of segmental instability than an increase in total range of motion. Segmental instability, therefore, is defined as a decrease in the capacity of the stabilizing system of the spine to maintain spinal neutral zones within physiological limits so there is no neurological deficit, no major deformity and no incapacitating pain. This definition emphasizes the quality as opposed to the quantity of motion and appears to fit the clinical observation that many patients with suspected segmental instability have greater difficulty moving in the mid-ranges of spinal motion than at the end-ranges. Panjabi s concept of neutral zone is the most currently accepted definition to explain instability and why it causes low back pain. The utilization of pedicle screws has increased reported fusion rates from 65% without instrumentation to 95% or greater with pedicle screw instrumentation The clinical success rates following lumbar fusion are generally reported to be between 50-70%. 9 Brox, et al. (2006) compared the effectiveness of lumbar fusion with posterior pedicle screws on 60 patients with low back pain following a previous discectomy. 10 The primary outcome measure was oswestry disability index and the success rate was 50%. Maghout, et al retrospective study of lumbar fusion outcomes among Washington State worker s compensation patients revealed an overall disability rate at two years following lumbar fusion of 63.9% and a reoperation rate of 22.1%. 11,42 Review of the literature by Turner, et al found no randomized trials in the literature on lumbar fusion, but their metanalysis found 68% of patients reported a satisfactory outcome. 45 These were all level III reports. Fritzel, et al conducted a multi-center randomized control trial comparing three techniques of lumbar fusion to non-surgical treatment. There were 294 patients, and 63% of patients receiving lumbar fusion reported being better or much better. Brox, et al. 2003, fusion patients reported improvements ranging from 36-49% on pain and disability scales. 10,17 Common complications of fusion reported by Deyo, et al include instrument failure 7%, complications at the bone donor site 11%, neural injuries 3% and failure to achieve a solid fusion or pseudoarthrosis 15%. 18 An assessment of spinal fusion by the Andalusian Agency for Health Technology Assessment (AETSA) (Martinez, Ferez, et al. 2009) concluded the available scientific evidence about spinal fusion is scarce and based on low to moderate quality studies. 19 The optimal rigidity of a pedicle screw based implant for achieving lumbar fusion has not been established. The same situation exists in establishing the optimal treatment for long bone 19

20 fractures. An increasing accepted inherent complication of rigid lumbar fusion is the development of adjacent level degeneration The reported prevalence of degeneration at the adjacent segments has ranged from 5-43%. 3,8 The prevalence of lumbar surgery performed for the treatment of adjacent level degeneration has ranged between 2-15%. 12 Ghiselli reported the rate of surgical intervention for adjacent level disease after a rigid lumbar fusion is 3.9% per year during the first 10 years following the primary lumbar arthrodesis. 12 In their series, a Kaplan-Meier survivorship analysis predicted at 10 years, 36.1% of the patients would have sufficient disease to warrant additional surgical intervention. Based on that data, 11 patients in this series would have been expected to undergone additional surgery for adjacent level degeneration. Only three patients have undergone an extension of their original surgery secondary to adjacent level degeneration. This data supports the concepts that a less-rigid fusion in the transitional zone between operated and non-operated levels may decrease the incidence in development of adjacent level degeneration versus a rigid lumbar fusion. 14 Limitations of this study are that the indication for surgery are multiple and do generate a confounding factor when comparing overall results (back pain, lumbar stenosis with instability, lumbar stenosis due to adjacent disease), lack of a control group for every tested variable, and low percentage follow up for the surgeries that occurred in Another limiting factor is that neither the fusion rate nor residual mobility at the operated levels was analyzed nor recorded among groups. This impossibilitates this study to clearly access any bone forming or adjacent motion preservation property this device may theoretically have. The perfect amount of motion needed to stimulate bone formation and not to alter the biomechanical properties of adjacent levels is yet to be determined. This study however shows us great clinical data. It tells us that patients can adequately be treated for fusion indications with the slotted dynamic device and an interbody fusion. Patients did better when interbody spacers added to the initial stability of the construct, especially when back pain was involved in the diagnosis. Reaching up to 65.6% of 15 point improvement on the ODI compared to 34% when cages were not used. The better results also occurred later in the series when the surgeon was more familiar with the system. Conclusions The DSS in association with interbody spacers seems to be adequate to stabilize the spine to treat stenosis with instability or discogenic back pain. Overall results are similar to those in the literature of rigidly instrumented spines. Adding stability by adding an interbody spacers, dramatically improved the clinical results and were considered advisable by the authors for some groups where more stability was needed, especially when treating back pain with or without neurogenic pain/deficit. The need for surgery at adjacent segments was smaller for the DSS group when compared to the rigid instrumentation literature. The addition of interbody devices to the construct seemed to have improved the percentage of 15 point improvement on ODI from less than 40% to more than 60% of patients. Out of the 239 patients, only 3 needed further surgery to treat symptomatic adjacent level disease. (Based on historical literature predictions, this series should have 11 patients that would need surgery for this reason if rigid implants were used). 20

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