25. Use of Precontoured Titanium Alloy Rods to Induce Thoracic Kyphosis after Sequential Posterior Release: a Cadaveric Spine Study

BACKGROUND CONTEXT Implications for failure to restore normal thoracic kyphosis (TK) in treating adolescent idiopathic scoliosis are just beginning to be understood. Techniques for releasing the rigid spine have included ligamentous releases and discectomy procedures, as well as bony resection; however, the extent these releases improve the ability for a precontoured rod to induce thoracic kyphosis has been lacking. PURPOSE Evaluate the ability of precontoured rods to induce thoracic kyphosis in a cadaveric spine and determine the effectiveness of sequential posterior releasing procedures. STUDY DESIGN/SETTING In vitro biomechanical study. OUTCOME MEASURES Sagittal Cobb measurement of T4-12 construct as well as superior, mid and inferior regions. METHODS Dissected 6 fresh thoracolumbar (T3-L2) spine specimens and acquired lateral and anteroposterior radiographs to: confirm all levels were mobile; exclude pathological conditions; and assess sagittal Cobb angle (T4-T12). Pedicle screws were placed from T4-T12 bilaterally. Intact status was defined by over correcting the initial sagittal cobb measurement and reducing the precontoured, bilateral titanium alloy rods. Cobb measurements were repeated following rod reduction. Removed rods and repeated over correction process for the following release procedures implemented sequentially: (1) interspinous and supraspinous ligaments (ISL) transection; (2) ligamentum flavum transection; (3) Ponte osteotomy; (4) posterior longitudinal ligament transection (PLL); and (5) lateral discectomy. Used a novel rod-bender to create consistent, over corrected contoured rods at each step. Collected data on the radius of each rod's curvature before and after each reduction. Cobb measurements collected on the instrumented construct for each procedure. Comparisons determined the effective contribution of each sequential release on the inducement of TK. RESULTS Initial sagittal kyphosis from T4-12 was 38° and kyphosis increased to 52° with over correction with the precontoured rods applied. Each release resulted in 5-7° of additional kyphosis, and largest releases were ISL and PLL. The ISL transection was not significantly different than the intact condition with rod correction. All subsequent releases resulted in significant (p<0.05) increases in correction compared to the intact with rod correction. Regionally, kyphosis increase following successive releases was consistent at approximately 2° for superior, middle, and inferior regions. Comparing the radius of the precontoured rod curvature before and after application showed loss in rod curvature following reduction independent of the number of releases. CONCLUSIONS Increased kyphosis was created in the thoracic spine using precontoured rods. Subsequent posterior releases provided a substantial, meaningful clinical change in the ability to induce additional kyphosis. Although each posterior release was effective for induction of TK (ISL and PLL having largest increase), ISL transection alone was not statistically different than the intact condition with rods applied. More than one release was necessary to induce a significant change in TK compared to the over correction accomplished on the intact thoracic spine. Rod flattening consistently occurred following attempted over correction with rod reduction, although the thoracic spine was sequentially destabilized with each release. Therefore, a presurgical plan for TK correction may be limited regardless of the number of releases performed. FDA DEVICE/DRUG STATUS pre-contoured titanium alloy rods (Approved for this indication). Implications for failure to restore normal thoracic kyphosis (TK) in treating adolescent idiopathic scoliosis are just beginning to be understood. Techniques for releasing the rigid spine have included ligamentous releases and discectomy procedures, as well as bony resection; however, the extent these releases improve the ability for a precontoured rod to induce thoracic kyphosis has been lacking. Evaluate the ability of precontoured rods to induce thoracic kyphosis in a cadaveric spine and determine the effectiveness of sequential posterior releasing procedures. In vitro biomechanical study. Sagittal Cobb measurement of T4-12 construct as well as superior, mid and inferior regions. Dissected 6 fresh thoracolumbar (T3-L2) spine specimens and acquired lateral and anteroposterior radiographs to: confirm all levels were mobile; exclude pathological conditions; and assess sagittal Cobb angle (T4-T12). Pedicle screws were placed from T4-T12 bilaterally. Intact status was defined by over correcting the initial sagittal cobb measurement and reducing the precontoured, bilateral titanium alloy rods. Cobb measurements were repeated following rod reduction. Removed rods and repeated over correction process for the following release procedures implemented sequentially: (1) interspinous and supraspinous ligaments (ISL) transection; (2) ligamentum flavum transection; (3) Ponte osteotomy; (4) posterior longitudinal ligament transection (PLL); and (5) lateral discectomy. Used a novel rod-bender to create consistent, over corrected contoured rods at each step. Collected data on the radius of each rod's curvature before and after each reduction. Cobb measurements collected on the instrumented construct for each procedure. Comparisons determined the effective contribution of each sequential release on the inducement of TK. Initial sagittal kyphosis from T4-12 was 38° and kyphosis increased to 52° with over correction with the precontoured rods applied. Each release resulted in 5-7° of additional kyphosis, and largest releases were ISL and PLL. The ISL transection was not significantly different than the intact condition with rod correction. All subsequent releases resulted in significant (p<0.05) increases in correction compared to the intact with rod correction. Regionally, kyphosis increase following successive releases was consistent at approximately 2° for superior, middle, and inferior regions. Comparing the radius of the precontoured rod curvature before and after application showed loss in rod curvature following reduction independent of the number of releases. Increased kyphosis was created in the thoracic spine using precontoured rods. Subsequent posterior releases provided a substantial, meaningful clinical change in the ability to induce additional kyphosis. Although each posterior release was effective for induction of TK (ISL and PLL having largest increase), ISL transection alone was not statistically different than the intact condition with rods applied. More than one release was necessary to induce a significant change in TK compared to the over correction accomplished on the intact thoracic spine. Rod flattening consistently occurred following attempted over correction with rod reduction, although the thoracic spine was sequentially destabilized with each release. Therefore, a presurgical plan for TK correction may be limited regardless of the number of releases performed.