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Thread: Tethers commonly break 1 to 2 years after VBT procedures

  1. #1
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    Tethers commonly break 1 to 2 years after VBT procedures

    Anterior Spinal Growth Tethering for Skeletally Immature Patients with Scoliosis
    RESULTS: Eight (47%) of the patients had a suspected broken tether.
    From the study of 17 patients:
    Broken Tether Suspected: 8 patients
    1 broken level: 7 patients
    3 broken levels: 1 patient

    Location of broken levels:
    T8-9: 4
    T9-10: 1
    T10-11: 3
    T11-12: 1
    T12-L1: 1

    Time period of tether breakage:
    1st erect to 6 months postop: 0
    6 months to 1 year postop: 0
    1 year to 18 months postop: 1
    18 months to 2 years postop: 3
    2 years to 2.5 years postop: 2
    2.5 years to 3 years postop: 1
    3 years postop: 1

    Commentary:
    Previous studies have shown that the PET cord has a tensile strength of 3,000 N, yet in the current study, the tether failed in nearly half of the cases. ASGT is proposed to work via initial tensioning on the convex side, with subsequent spinal growth creating further tension until convex vertebral end-plate growth is inhibited via the Hueter-Volkmann law. Tether breakage typically occurred at the levels below the apex, with the most common broken levels being T8-9 and T10-11 (range, T8-9 to T12-L1). Interestingly, none of the cases had 3 adjacent screws simultaneously splayed due to failure. This would likely be the case if the tether failed beneath the locking set screw, an obvious stress riser. In the 2 cases of tether failure confirmed intraoperatively, the breakage occurred between the screws, not under the set screws. Until the screw angulation data were analyzed, the increased divergence of the screws and implant failures were not recognized except in the most severe example. Considering this, it is important to measure the segmental angle between each pair of screws, as widening of this angle appears to be an early indication of implant failure. In some cases, recognition of tether breakage may actually prevent further intervention. In at least 1 case in the current study, there was initially some concern about overcorrection prior to the observed widening between the screws at 1 level. After tether breakage, the progression of the correction slowed to the point that implant removal for overcorrection was unnecessary. The exact cause of the failure remains unclear; however, because of the greater motion in the lower thoracic region, this area seems to be at higher risk. A more robust implant would be ideal, although an implant with a fatigue life of decades rather than years seems unlikely. Further development is necessary.
    Last edited by Dingo; 10-04-2018 at 12:56 PM.

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    Also from the study some interesting commentary.

    Radiographic and Clinical Measurements
    Preoperatively, all (100%) of the patients were at Risser stage 0; the triradiate cartilage of the acetabulum was open in all but 1 patient. The mean bone age was 11.8 ± 1.7 years. Hand radiographs were available for Sanders staging for 94% of the cohort, with all patients in stages 1 to 4. The patients, on average, grew 15.2 ± 5.4 cm over the study period (mean height, 150.6 ± 9.1 cm preoperatively and 165.8 ± 8.5 cm at latest follow-up). Many still had substantial growth remaining at latest follow-up, with 47% at Risser stage 0 or 1 and triradiate cartilage closed in all but 3 patients. Preoperatively, the mean thoracic curve magnitude was 52° ± 10° (range, 40° to 67°), with 48% ± 14% flexibility on side-bending radiographs. The mean lumbar curve was 33° ± 10°. Both the thoracic and lumbar curves showed significant initial correction (p ≤ 0.001) and continued to decrease until ;18 to 24 months, before increasing slightly over the remainder of the study (Fig. 1 and Table I). At the latest follow-up, the average percent correction of the tethered thoracic curve was 51% ±35% (range, 5% to 118%). Significant reductions were seen from the preoperative to the first postoperative evaluation in the coronal plane (upper thoracic curve and thoracic curve, p ≤ 0.001; lumbar curve, p = 0.001) and in the thoracic ATR (p = 0.043) (Table I). This correction was maintained at the latest follow-up. There were no significant differences in T2 to T12 kyphosis or lumbar ATR from the preoperative to the first postoperative evaluation (p = 0.17 and p = 0.18) or from preoperatively to the latest follow-up (p = 0.266 and p = 0.093). Additionally, no significant differences were found in any deformity measurements between the first postoperative evaluation and the latest follow-up (p > 0.3 for all).

    Discussion
    The average preoperative thoracic curve among our cohort was 52°, which decreased to 31° by the first postoperative visit. This initial decrease was due to the tension applied to the tether with correction through the compressed convex-side disc spaces. On average, progressive correction was gained during the first 24 months after the initial procedure, particularly during the 6-month to 1-year time period. After 18 months, however, the results were less consistent, with some curves beginning to progress, some continuing to correct, and some overcorrecting. The 1 to 2-year time period seemed to mark a key point, with 4 tethers breaking, 3 patients undergoing tether removal or revision, and 1 patient having continued lumbar curve progression to the point of requiring posterior spinal fusion. It is essential that future studies capture longer-term follow-up after ASGT, as the outcomes appeared to be more varied beyond 18 months. In general, the average correction observed in our study was not as substantial as that in previous reports. This difference may be related to our cohort having a larger average preoperative thoracic curve (52° versus 44°) with less flexibility (48% versus 57%) than the 2-year cohort described by Samdani et al.. Given the relatively young age and remaining growth of our cohort at surgery (94% with open triradiate cartilage), there is both greater potential for correction and greater potential for adding-on or progression of the deformity.
    Last edited by Dingo; 10-04-2018 at 03:52 PM.

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    Commentary on Dr. Newton's study from John A Herring, MD

    Newton et al. reviewed a series of cases in which flexible anterior vertebral body tethering was used in the treatment of growing patients with scoliosis of varying etiologies. There is currently much interest in attempts to treat progressive scoliosis without bracing or fusion by altering natural vertebral growth in order to progressively correct existing deformity. This is analogous to the use of epiphyseal-spanning plates to correct angular deformities of the extremities. Stapling across disc spaces has been reported to produce growth correction, and this report deals with a flexible cable device anchored with screws to the vertebral bodies.

    Tethering was considered for skeletally immature patients with progressive scoliosis who had a primary thoracic curve magnitude of >40° and who were at Risser stage 0; all but 1 patient had open triradiate cartilages. The authors’ results from this study of 17 cases were mixed and raise a number of questions. On the favorable side, 10 of the 17 cases were considered clinically successful, with gradual improvement in the average Cobb angle from 52° to 24° over 2 years of follow-up. Coronal curve correction was attained without significant change in sagittal curvature. This, along with prior animal studies by the lead author and others, validates the concept of growth-induced correction of deformity. On the negative side, within a relatively short period of follow-up, there were 4 patients whose curves progressed to surgical levels, and there was some overcorrection. Of major concern was breakage of the tether in several cases.

    A number of issues require further research. First, the indications for surgery must be better defined relative to both curve severity and patient skeletal maturity. If the patient is too immature, curves will overcorrect. With larger, stiff curves, there may be failure to correct or continued progression.

    Second, the nature of the tether must be studied. Unknowns include the exact force needed to alter the growth of the vertebrae and the ideal tensile strength of the tether along with the fatigue resistance at the interface of the tether and the vertebra. Prior studies show continued mobility between tethered segments, so it is likely that the tether will be continuously fatigued, perhaps for the lifetime of the patient. The third unknown is thus the fate of the spine with residual, untethered curvature: is progression likely to occur?

    It is very important for the spinal-deformity community to give these and others who are pioneering this research time to answer these important questions before “jumping on the band wagon” and performing many surgical procedures. The follow-up of these patients is quite limited and many shortcomings are likely to surface in the near future. We should let these respected researchers continue their careful work before encouraging widespread clinical application.

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