Arch Orthop Trauma Surg (2001) 121 : 142–147

© Springer-Verlag 2001

O R I G I N A L A RT I C L E

K. Birnbaum · C. H. Siebert · J. Hinkelmann · A. Prescher · F. U. Niethard

Correction of kyphotic deformity before and after transection of the anterior longitudinal ligament – a cadaver study

Received: 29 February 2000

Abstract With a custom-made measuring unit, two separate experiments, involving six and five cadaveric torsos with intact rib cages and sternums, respectively, were carried out to determine the effect of the transection of the anterior longitudinal ligament with and without osteodiscectomy and its influence on the thoracic kyphosis. The open or thoracoscopically assisted anterior release, as part of the operative treatment of scoliosis or kyphosis, usually consists of a transection of the anterior longitudinal ligament (ALL) and an additional discectomy. A complete osteodiscectomy, however, is not always possible with a minimally invasive approach. As part of our biomechanical research, we attempted to quantify the amount of correction achievable with a defined force prior to and following the isolated transection of the anterior longitudinal ligament. The aim of the study was to clarify whether or not an isolated transection of the anterior longitudinal ligament is sufficient to obtain an adequate anterior release of the spine. In the surgical treatment of kyphotic deformities, anterior release of the spine is performed in the form of a transection of the ALL and discectomy. Recently, video-assisted thoracic surgery has become increasingly popular in spine surgery. As part of this change in surgical technique, the question has arisen as to what extent an isolated transection of the ALL provides an adequate release of the thoracic spine. Eleven human spines were retrieved from fresh cadavers, dissected, and attached to a specially constructed apparatus. The spine was attached to the construct at the twelfth vertebral body. C6 and C7 were fixed in synthetic resin. We installed the instruments in such a manner as to reproducibly apply a torsional moment of 10 Nm to the spine. Motion was only permitted in the sagittal plane. K. Birnbaum · C. H. Siebert · J. Hinkelmann · F. U. Niethard Orthopedic Department, Pauwelsstraße 30, 52074 Aachen, Germany e-mail: [email protected] Tel.: +49-241-8088559, Fax: +49-241-8888453 A. Prescher Anatomical Institute, University Hospital, Technical University Aachen, Germany

Segmental transections of the ALL were carried out from T3 to T7. For comparison, the sagittal Cobb angle was also documented following an anterior release combined with an osteodiscectomy. With the isolated transection of the ALL, an average correction of the sagittal Cobb angle of 4° in each functional spinal motion segment was recorded. In comparison, the additional osteodiscectomy led to a further average increase of only 2° per level. The measurements performed on human cadavers showed that the isolated transection of the ALL leads to a sufficient anterior release of the thoracic spine, allowing a correction of the kyphotic deformity. The release with a concomitant osteodiscectomy represents a more time-consuming and more invasive procedure resulting in only a slightly greater amelioration of the sagittal Cobb angle, while being associated with a greater patient morbidity. Keywords Spine · Thoracic spine · Hyperkyphosis · Anterior longitudinal ligament · Biomechanics

Introduction The proper operative management of kyphotic deformities of the spine caused by various underlying diseases still leads to heated debates among orthopedic surgeons. A massive kyphotic deformity is generally treated surgically in an attempt to achieve a stable correction and to avoid further progression of the deformity [9, 14, 15]. In the past, the operative management has frequently failed because of a lack of understanding of the underlying pathology and problems with the operative procedure itself. With regard to the operative management of the kyphotic deformity, a two-stage treatment protocol, including a primary thoracotomy and anterior release including an osteodiscectomy followed by an instrumented dorsal stabilization, has generally been accepted [9, 14, 15]. Bradford et al. [2] was one of the first to recognize that successful surgical management of the kyphosis depended on the correction of the anterior column. Griss [8] sup-

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ported this view and recommended, in accordance with other authors [7, 10, 25], a combined anterior and posterior procedure for the management of a kyphotic deformity. Experience has shown that the transthoracic approach is associated with various risks and complications [1, 17, 22]. The advantage of the thoracoscopic procedure over the standard thoracotomy is its minimally invasive nature utilizing only small portals. Once the learning curve for the thoracoscopic procedure has been surmounted, shorter surgery times and reduced patient morbidity can be obtained [5, 13, 16]. Hammerberg [9] pointed out that a progression of the kyphotic deformity during adequate conservative management, significant symptoms, as well as the development of neurological deficits can be considered an indication for surgical intervention. In a pediatric population, surgery should only be considered once a Cobb angle between 70° and 75° has been documented. Heine et al. [10] defined an angle of greater than 70° combined with irrepressible pain as absolute indications for surgery, while a Cobb angle between 50° and 70° was viewed as a relative indication in a juvenile population. The thoracic, thoracolumbar and lumbar kyphosis associated with Scheuermann’s disease and its growth disturbances of the vertebral body are generally the most common indications for surgical intervention. If the kyphosis is not corrected, the spinal deformity will tend to progress, thereby displacing the weight-bearing axis further anteriorly, leading to an increased excentric loading of the spine [14]. It was the goal of this study to answer the question of whether or not isolated transection of the ALL provided sufficient anterior release to allow for an adequate correction of the kyphosis compared with the degree of correction achieved by a ligament transection combined with an osteodiscectomy.

Fig. 1 The apparatus with metal base plate and fixation arm for T12

Materials and methods During the period from May 1997 to August 1998, 11 fresh-frozen human cadavers were evaluated in the Anatomical Institute of the Technical University of Aachen. The torsos were dissected, and the intact spine including its musculature, rib cage and sternum was retrieved during the first 24 h post-mortem. The spinal specimen was isolated from C6 to L1 and stored at –20 °C. One day prior to testing, the specimens were thawed to room temperature. It has been shown in previous studies that this type of storage does not adversely affect the physical properties of bone [21], of the annulus fibers of the intervertebral disc [11]or of the longitudinal ligaments [23]. The anterior release in the form of an isolated transection of the ALL at the level of the intervertebral discs was carried out on 5 caderveric thoraces. Comparative analysis of the effect on the Cobb angle of the combination of transection of the ligament and an osteodiscectomy was performed on another 4 fresh-frozen human cadavers. The thorax was dissected free of unnecessary soft tissues. C6 and C7 were fixed in Technovit, a synthetic resin, to allow for the application of 10 Nm torsion moment during the test. The specimen was attached horizontally to the measuring apparatus at T12. The apparatus consisted of a metal base plate and a fixation arm (Fig. 1). The plate was securely fastened to the bone with the help of four cortical screws. This ensured a suitable fixation of the spines in the constructed frame.

Fig. 2 Torsion unit permitting the defined application of 10 Nm to the spine specimen. A total of 6 extensometers were applied in such a manner that the torsion and bending moments were accurately measured. The extensometers are connected in standardized fashion to form a Wheatstone bridge and were temperature-compensated

At the level of C7, the torsion unit was attached. With two universal joints, a torsional moment of 10 Nm could be applied to the specimen in a reproducible fashion. With this construction, we were able to prohibit forces with other directions beyond the sagittal plane influencing the movement of the thoracic spine. The reproducibility of the force development was documented by six ex-

144 vertebral bodies under consideration of the proximal and distal neutral vertebral body. The evaluation was continued to allow for an immediate comparison of the results. The classic anterior release was completed by carring out the additional osteodiscectomy, and a second series was measured. A force of 10 Nm was again applied, and the change in the Cobb angle was documented. These values were compared with those measured with the transection of the ALL alone. At each level, the measurements were repeated three times following the transection of the ALL in the first phase, as well as after the osteodiscectomy during the second phase, to document the changes in the Cobb angle.

Fig. 3 Schematic drawing of the Wheatstone bridge

Fig. 4 Horizontal positioning of the thorax with the help of a fixation arm with free range of motion in the sagittal plane (view from above)

tensometers, which were applied in such a manner that only torsional moments and bending stresses were measured (Fig. 2). Before testing, a standarized calibration was performed. To ensure a standardization of the measuring technique, the six extensometers were connected to a Wheatstone bridge and were temperature compensated (Fig. 3). The specimen was attached to the outrigger horizontally allowing for unimpeded movement along the sagittal plane (Fig. 4). With this measuring unit, it was possible to apply the force manually, while allowing the examiner to monitor the changes on the instruments continuously. In a step-by-step fashion, the second examiner transected the ALL one level at a time from T3 to T7. As part of the second series, an additional osteodiscectomy was carried out at the same levels. The anterior release of the cervicothoracic junction was carried out prior to the testing to avoid incidental loading of the thoracic spine. The osteodiscectomy was performed through an anterolateral incision of the annulus. The disc was completely removed, with the dissection being extended to the posterior longitudinal ligament. The procedure was carried out using standard thoracoscopic instrumentation and long dissectors. The force the surgeon was reliably able to assert on the spine with the help of a distraction device was found to be 10 Nm prior to testing. Therefore, this force was used to achieve the ‘intraoperative’ correction of the kyphosis. The force of 10 Nm was applied on the specimen prior to the ligament transection, and the degree of correction was documented. The ALL of the thoracic spine was then transected one level at a time beginning at the intervertebral space between the 3rd and 4th vertebral body. During the evaluation, the force of 10 Nm was reapplied to the spinal specimen and the Cobb angle measured following each level of transection. The Cobb angle was measured with a goniometer placed directly on the

Results The age of the donors of both sexes ranged between 54 and 92 years (average age at the time death: 65 years). Standard anteroposterior (AP) and lateral X-rays were made of all specimens to define degenerative changes of the spine. The evaluation of the medical histories was not possible, so that all spines with evidence of prior spinal or thorax surgery were excluded from the study. None of the individuals presented with a clinically apparent deformity of the region of interest, except for one minor scoliotic deformity. The specimens with degenerative changes, especially in the form of ossifications of the ALL were excluded from the biomechanical measurements to avoid falsification of the data. This was the case in specimen number 9. Specimen number 1 had to be excluded as well, because of changes associated with an intrathoracic tumor-like lesion, which prohibited the endoscopically assisted release. The average sagittal Cobb angle of the spine with the intact rib cage prior to any intervention was found to be 38°. The angle was measured between the end vertebrae of the curve. In most cases, the top and the lower end vertebrae were T1 and T12 or L1, respectively. We put a Kirschner’s wire parallel to the inferior endplate of the lower end vertebrae and one parallel to the top of the superior end vertebrae. A right angle is then erected on these wires with a second Kirschner’s wire, so that a direct measurement of the Cobb angle by a goniometer was possible. In this way, we could measure the angle continuously during the transection of the ALL. We measured the angle from the baseline measurement to the transection of the ALL, the isolated application of a force of 10 Nm applied at the proximal end of the specimen through the stabilized C6 and C7 was done to measure the amount of ‘spontaneous’ correction. The force led to a total correction of 16°, on average, resulting in a reduction of the Cobb angle to 22° (Fig. 5; Tables 1, 2, 3). The isolated transection of the ALL led to an average correction of the kyphosis documented in the form of a reduced Cobb angle of 4° per motion segment. One must take into account that there is a greater amount of correction of the Cobb angle possible associated with the anterior ligamentous release in the more proximal segments (down to T5) than in the lower thoracic spine. From a biomechanical standpoint, the upper thoracic spine is therefore viewed as part of the cervicothoracic junction.

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Fig. 5 Diagram depicting the changes in the Cobb angle following transection of the anterior longitudinal ligament (ALL). Negative scale for the change of the Cobb angle before transection of the ALL with a standardized force of 10 Nm

Fig. 6 Diagram depicting the changes in the Cobb angle following transection of the ALL combined with osteodiscetomy

Table 1 Transection of the ALL of the thoracic spine

Discussion

Motion segment (ms)

Specimen 2

Specimen 4

Specimen 6

Specimen 8

Specimen 11

10 Nm ms 3-4 ms 3-5 ms 3-6 ms 3-7

13° 5° 4° 2° 2°

19° 7° 3° 5° 3°

19° 6° 3° 3° 1°

14° 7° 5° 5° 2°

15° 5° 5° 5° 2°

Table 2 Transection of the ALL and additional osteodiscectomy Motion segment (ms)

Specimen 3

Specimen 5

Specimen 7

Specimen 10

10 Nm ms 3-4 ms 3-5 ms 3-6 ms 3-7

19° 11° 9° 9° 7°

16° 8° 6° 5° 4°

13° 6° 4° 4° 2°

16° 7° 5° 6° 3°

Table 3 Average amelioration of the Cobb angle after transection of the ALL and after additional osteodiscectomy Motion segment (ms)

Transection of the ALL

Transection of the ALL and additional osteodiscectomy

ms 3-4 ms 3-5 ms 3-6 ms 3-7

6° 4° 4° 2°

8° 6° 6° 4°

An additional osteodiscectomy in each motion segment led to only a slight further amelioration of the Cobb angle (on average 2°), so that a total correction of 6° was documented for each motion segment following the application of the standardized force of 10 Nm. The correction of the Cobb angle obtained after anterior ligamentous release and osteodiscectomy are summarized in Fig. 6 and Tables 2, 3.

With the help of this biomechanical analysis of the average correction of the sagittal Cobb angle following the isolated transection of the ALL, we were able to document an average reduction of the kyphosis of 4° per transected spinal motion segment. The additional osteodiscectomy at the corresponding levels led to a further reduction of Cobb angle of only 2° per released level. This cadaver study therefore seems to imply that an isolated transection of the ALL, which could be carried out in a minimally invasive manner with the help of thoracoscopy, would provide enough of an anterior release to allow for adequate correction of the kyphotic deformity with dorsal instrumentation. Based on the data presented by Neumann et al. [18] with regard to the biomechanical qualities of the ALL, the increased flexibility of the anterior column of the spine following complete transection of the ALL was to be expected. Compared with the posterior longitudinal ligament, the ALL has a higher tensile strength and possesses an almost inseparable bony attachment to the upper and lower vertebral bodies to guarantee tensioning of the anterior column of the spine [18]. The average initial sagittal Cobb angle documented a kyphosis of 38° in this series, which exceeds the normal values described in the literature. This may have its origin in the high average age of the cadavers (63 years) and the secondary, age-related hyperkyphosis of the thoracic spine. The advantage of the minimally-invasive anterior ligamentous release is seen in the fact that the procedure can, after a short learning period, be performed quickly and safely. Only minor demands are placed on the instrumentation, requiring no special equipment other than long dissectors, scissors, and retractors. The operation can be carried out as part of a video-assisted thoracic surgery. The classic open anterior release including osteodiscectomy is a more complicated, more invasive, and more time-consuming procedure than the reported isolated release. With the help of the video-assisted anterior ligamentous release,

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a combined AP fusion could be performed in one operative session. This type of approach seems feasible, since biomechanical investigations show that the isolated anterior ligamentous release (transection of the ALL) leads to a sufficient mobilization of the spine comparable to that achieved with the additional osteodiscectomy. In a range of animal experiments, Cunningham (1998) has demonstrated that the thoracic interbody spinal fusions performed by thoracoscopy are equivalent from histologic, biomechanical, and radiographic viewpoints to those performed by a thoracotomy. Concerning the intraoperative complications, we found different results in the literature. Cunningham (1998) noted intraoperative complications in his endoscopy group, leading to longer operative times, higher estimated blood loss, and increased animal morbidity, indicating a substantial learning curve associated with the adoption of this surgical technique. In contrast, most of the authors came to the conclusion that the endoscopic procedure is associated with a shorter hospital stay, limited blood loss, and less postoperative discomfort [13, 19, 20]. Rothenberg et al. [20] has already used the thoracoscopic approach for discectomy and anterior release of two to nine vertebral levels for children (n = 20) with idiopathic and neurogenic scoliosis or kyphosis. All of the endoscopic procedures were completed successfully with less blood loss, less pain and morbidity than with the open technique. Similar to these investigations, the findings of Huntington et al. [13] indicate that the thoracoscopic discectomy technique is equivalent to the open technique and support the efficacy of the thoracoscopic technique for anterior spinal fusion. Video-assisted-thoracoscopic surgery for anterior release of the spine has significant advantages over the standard thoracotomy, including reduced incisional pain and avoidance of the postthoracotomy pain syndrome [3]. The ideal indication for minimally invasive ‘anterior ligamentous release’ seems to be juvenile kyphosis. In addition to Scheuermann’s disease with a sagittal Cobb angle greater than 70°, congenital kyphosis or malformation kyphosis associated with a lack of segmentation and/or hemivertebra with a Cobb angle greater than 70° in the child older than 3 years of age are indications for such an anterior ligamentous release [25]. Generally, the ALL should be transected at the apex of hyperkyphosis. The complete separation of the ALL over a stretch of three to four segments seems to be sufficient in order to achieve flexibility of the anterior column of the spine. Animal studies have shown that the thoracoscopy-assisted anterior release plus osteodiscectomy leads to results comparable to those achieved with the open procedure with regard to the flexibility of the thoracic spine [11, 21, 24]. Our biomechanical measurements show that isolated transection of the ALL leads only to an insignificant reduction of the achievable correction of the kyphosis (-2° per spinal motion segment) compared with transection plus additional osteodiscectomy: The required enlargement of the procedure to achieve this additional correction, therefore, does not seem justified in all cases.

Contradictory to the results of Feiertag et al. [6], our research showed an average change of the Cobb angle of 4° per completely released motion segment. In patients with a transection at the level of four motion segments, a reduction of the kyphotic angle (measured by the Cobb angle) of 16° would result following the introduction of a compressive force of 10 Nm. Feiertag et al. [6] reported only an increased motion of 3° secondary to a osteodiscectomy, laminectomy, and resection of capitular articulation of the rib. The described isolated anterior ligamentous release is a procedure that can be performed as a video-assisted thoracoscopic operation, reducing the risk of a secondary postthoracotomy-syndrome thanks to its minimally invasive nature with reduced surgical trauma. In all, the hospital stay and associated costs can be reduced. Our experience has shown that following a short ‘learning curve’, the thoracoscopy-assisted complete separation of the ALL at three to four levels can be completed in about 30 min utilizing three portals. The resectability of the ALL is generally facilitated by the fact that it is slack over the discus intervertebralis, rather than firmly attached to it. We have come to the conclusion that ossifications of the ALL should be considered a contraindication for isolated anterior ligamentous release, because the resulting rigidity does not permit sufficient correction. To what extent an indication can be seen for scoliotic deformities could not be clarified, due to the fact that no specimen with a higher grade scoliosis was analyzed. Risk factors for patients with the video-assisted thorascopic spinal technique include chronic obstructive pulmonary disease (COPD), pleural adhesions, and spinal tumors [12]. Pleural adhesions caused by this technique have not been described. We cannot yet make a statement concerning the longterm effect of an isolated transection of the ALL. The operation technique described guarantees a sufficient anterior release of the thoracic spine from the biomechanical point of view. This allows in our opinion a complete erection of the thoracic spine through the subsequent instrumented dorsal spondylodesis. To what extent isolated transection of the ALL leads to a reduced fusion rate and stability of the thoracic spine could not be answered with our biomechanical investigations. Nevertheless, the results demonstrate that an additional osteodiscectomy is not required to achieve adequate/ anterior release of the thoracic spine.

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