In spite of much promise from principles of three dimensional reduction of scoliotic deformities, as illustrated by CD technique, the world of scoliosis continues to lead lively theoretical discussions around many questions. What are the best techniques of reducing scoliotic deformities? Which techniques are the most efficient and least dangerous? These are but a few commonly debated questions. In this domain, the affirmations remain more numerous than the scientific proof, yet many still passionately defend their point of view. At the heart of this discussion are the theoretical aspects, but often the practical and technical aspects may be neglected. Our purpose, therefore, is not to add a new theory to the treatment of scoliosis, but to show the practical aspects of the new CD Horizon technique. This article focuses on the technical aspects and clinical use of the CD Horizon system. We do not intend to exhaust the subject matter of scoliosis, nor are we representing the only surgical technique for the CD Horizon. We feel however that the presented technique takes full advantage of the ancillary instruments and yields the best results with optimal safety. The goal of this article is to point out some of the theoretical principles of our philosophy for preoperative planning in scoliosis surgery. The technical aspects are detailed in order to present our point of view regarding the best way to take advantage of the new CD Horizon system. It is however strongly recommended not to apply the presented method as a cookbook recipe. Appropriate training in scoliosis surgery and sufficient personal experience with spinal surgery is mandatory before applying the presented surgical technique.

PRINCIPLES OF PRE-OPERATIVE PLANNING

Although the principles have been extensively reported elsewhere, we feel it is important to re-emphasize a few of them. First, one must define the goal of surgery. The primary goal should be to re-establish three dimensional balance of the vertebral column. In practical terms, a reduction of the scoliosis should yield a residual curve with a balanced sagittal profile as near normal as possible. Obviously, the overcorrection of a curve which involves a final imbalance or a correction of a curve in only a single plane is undesirable. When imbalance or correction in just one plane occurs, one should clinically analyze the AP and lateral radiographs to avoid repeating the error. A common error is to focus on the most complete correction on the AP radiograph only: pursuing this goal through pure distraction only leads to an imbalanced sagittal contour. One has to realize that surgery does not address the normal vertebral bodies, but instead addresses the vertebral bodies deformed by the scoliotic torsion. Anatomic studies of scoliotic specimens have shown (personal data, publication in progress), among other things, that the anterior vertebral line is approximately 1/3 longer than the posterior vertebral line. One can imagine, therefore, the enormous traction that is exerted on the spinal cord in attempt of a total correction.

EXTENT OF THE CONSTRUCT

One should go back to the basic principles of the CD technique, however, certain points of detail are reviewed and emphasized here.

SUPERIOR LIMIT

A frequent error is to carelessly disregard the structural characteristic of the upper thoracic curve in a double thoracic scoliosis. (fig. 1a,b,c) Dynamic radiography in lateral flexion will permit one to foresee the superior extent of the construct. We measure the angle formed by the inferior endplate of the inferior most vertebra of the curve and the superior endplate of T1 on the bending film in which the patient bends toward the convexity of the upper thoracic curve. (fig 1b) This allows an estimation of the reducibility of the upper curve. We also measure the angle formed by the inferior endplate of the apical vertebra of the primary curve and the superior endplate of the most superior vertebra of the primary curve. This measurement is also made from the film in which the patient bends maximally toward the convexity of the curve, and it, too, determines flexibility. (fig. 1c)

Figure 1a

Figure 1b

Figure 1c

Fig. 1A: double thoracic scoliosis
Fig. 1B: bending film of the upper curvature to determine its reducibility: a’
Fig. 1C: bending film of the main curvature: b’; clinically satisfactory result if a’ - b’ <= 17°

For the balance of the shoulders to be satisfactory, the difference of the two aforementioned angles should not exceed 17 degrees. In our experience, this empirical rule works well. If, however, the superior curve is stiff, we use a sequential construct consisting of three rods; the rotation of the first rod to correct the first principle curve, the rotation of the second rod to correct the superior curve on the concave side, and complete stabilization with the third short rod on the convex side of the superior curve to connect to the first rod by a connecting tube.(fig. 2a,b,c,d).

Figure 2a

Figure 2b

Figure 2c

Figure 2d

Fig. 2A,B: Correction of a double thoracic curve with CD Horizon
Fig. 2C,D: Correction of the thoracic lordosis through rotation of the lower concave rod

INFERIOR LIMIT

Much has been written about the famous inferior limit. Of particular concern are double thoracic and lumbar curves. Many theories exist, such as the stable vertebra of King. Their value in the realm of 3D reduction remains controversial. Once again, this demonstrates the difficulty in establishing a general rule in the domain of scoliosis, and therefore, each curve must be examined according to its specific behavior. We do not utilize King's rule because this rule applies in our opinion to a correction by pure distraction. The curvilinear translation applied on the vertebral column during the « derotation » maneuver does not allow the application of this rule. This translation movement is very powerful and surpasses the possibilities of correction by simple distraction in idiopathic scoliosis. The importance of the reduction of the thoracic curve may overpower the reducibility of the lumbar curve. In this case, the resulting imbalance with translation of the thoracic curve will be progressively corrected through an accentuation of the lumbar curve. This phenomenon is the natural reaction of the organism intending to reestablish the trunk balance (and in our opinion falsely called "decompensation"). Such an overestimation of the flexibility of the lumbar curve should be avoided: true structural lumbar curves with important vertebral rotation should be instrumented down to L4. Persisting lumbar imbalance after a limited fusion may preserve some motion segments for a while. The danger however is the progression of degenerative changes at adult age which may need extensive revision surgery. Only if a limited fusion satisfactorily restores balance in both the coronal and sagittal planes, then this it should be carried out. Occasionally, it may be necessary to add a protective brace for the inferior uninstrumented curve for a number of months. Often an initial slight decompensation will spontaneously correct in four to five weeks. One must carefully study where the proposed construct will end. This includes evaluating the lateral view to look for a possible junctional kyphosis. In all cases, look for the chance to limit the extent of the construct, in order to save mobile segments. However, saving mobile segments must never be detrimental to the overall final balance of the spine (Fig. 3A - F). We are often surprised by the ability of the spine to compensate if the spinal balance has been restored even after an extensive fusion (Fig. 5A -C). Generally, the construct should include all of the rotated vertebra and span as far as the neutral vertebra. The behavior of the last vertebra to be instrumented must be studied carefully on both the lateral and bending AP radiographs. We require the disc space directly inferior to the last instrumented vertebra to be fully opened on the side bending films. This will ensure that the vertebral fusion is on top of a mobile balanced spine. It is also important to screen for an anatomic configuration that we call an "unstable vertebra". This vertebra is delimited by disk spaces which are pinched on opposite sites. A special claw configuration (« inverting claw ») allows the vertebra to act in the direction of the rehorizontalization of the last fused segments (Fig. 6A -C). This instrumentation is very solid and a lordosing effect of the side of the reducing rod is very favorable for sagittal balance(Fig. 7A-C). We prefer this instrumentation to the classic construct (a sublaminar hook on the reduction rod) each time that it is possible.

figure 3a	figure 3b	figure 3c
figure 3d	figure 3e	figure 3f

Fig. 3A,B: Initial correction of a triple curve with a short thoracic construct
Fig. 3C,D: Progressive junctional kyphosis during follow-up (note that the distal hook are not dislodged)
Fig. 3E,F: Extension of the construct reestablishing the spinal balance

figure 4a

figure 4b

figure 4c

figure 4d

Fig. 4A,B: Correction of a double major curvature above an L5S1 spondylolisthesis in a marfanoid patient
Fig. 4C,D: Correction of the sagittal imbalance with a short construct (CDI). Spondylolisthesis asymptomatique at 5 year f/u

figure 5a

figure 5b

figure 5c

Fig. 5A,B,C: Excellent mobility after posterior spinal fusion T5L3

figure 6a

figure 6b

figure 6c

Fig. 6A: « inverting » claw
Fig. 6B,C: inverting claw in a thoracic construct

PREPARATION AND POSITIONING OF THE PATIENT

The use of traction during the operation seems to us to facilitate the operation itself. In effect, the use of traction during the operation allows stabilization of the vertebral column at the time of surgery, but it also loosens up the spine through its viscoelastic behavior. Also, the placement of hooks and rod introduction is largely facilitated by this traction. It is very important to release this general distraction at the time of the rotation of the correction rod during surgery to avoid flattening of the sagittal curve by longitudinal distraction. In certain cases, it is preferable to use a very moderate amount of traction so as to leave a persistent coronal plane curve. This more pronounced remaining curve allows better correction of the thoracic lordosis during the rotation of the reducing rod. In general, the traction does not exceed 25% of the body weight, and in our experience this does not cause neurologic injury. Traction is applied through a padded occipitomandibular collar in the idiopathic scoliotic patients and by a halo in the neuromuscular scoliotic patients. Use of a halo in patients who have a short neck, also facilitates the work of the anesthesiologist. Placement in the prone position on a traction table (Cotrel table) must be done very carefully. The abdominal wall must be perfectly free of pressure to avoid any compression on the vena cava. We avoid hyperabduction of the arms which would place excessive stretch on the brachial plexus. The legs are, in the majority of cases, put in line with the trunk in order to facilitate the correction of lumbar hypolordosis or kyphosis. The skin incision is a straight line, although it will not be directly over all of the spinous processes due to the scoliotic deformity. The incision can be extended distally, in the cases in which fusion will be to L2 or L3, so that an iliac corticocancellous bone graft can be obtained by tunneling subcutaneously and thus avoiding a second incision. Preparation of the posterior arches includes careful removal of all soft tissue that covers them. In a primary surgery, we use only a Cobb elevator in order to preserve the osteogenic abilities of the periosteum and to limit blood loss by my meticulous subperiosteal dissection. Excision of the facet joint capsules is often done by electric cautery, which is also used to prepare the transverse processes. In final preparation, the entire posterior arches including the transverse processes need to be perfectly void of all soft tissue because the quality of the fusion and, therefore, the quality of the final long-term result depend upon it.

PLACEMENT OF HOOKS

The placement strategies of hooks is the same as with the classic CD technique. We use two hooks in a distraction configuration on the thoracic concave side in order to obtain a kyphosis effect during the rotation of the contoured rod.

INFERIOR LIMIT OF THE CONSTRUCT

As mentioned already about the most proximal aspect of the construct, it is desirable each time that it is possible to use a configuration called a "inverting claw". This solid fixation is very solid. On the side of the correction rod is an infralaminar hook and a supralaminar hook so that a lordosing effect occurs during rotation at the inferior limit of the construct. This also prevents a junctional kyphosis under the construct. The placement of the infralaminar hook is simple. The ligamentum flavum under the lamina is stripped to help expose the inferior lamina border. In certain cases, especially at the lumbar level, the lamina are very vertical and this causes risk of the lamina hook slipping. It is preferable in this case to use an oblique lamina hook. On the opposite side, we use at the last level of instrumentation, two hooks which are in opposite directions: one to be supralaminar, and the other infralaminar at the level of the same disc space. Sometimes we have to thin slightly the adjacent superior lamina to obtain better alignment of the two opposite hooks. Concerning the removal of the lamina, this must be generous to permit the proper manipulation of the hooks during the introduction of the rod. Associated with the lordosing claw of the other side, this configuration "inverting claw" permits the possible rehorizontalization of the last disc space.

figure 7a

figure 7b

figure 7c

figure 7d

Fig. 7A,B,C,D: 3D correction of a thoracic scoliosis with CD Horizon

THE SUPRALAMINAR HOOKS

Placement of supralaminar hooks must be done with great caution because of the space-occupying nature that they impose in this area of the spine. We first start with removal of the concerning articular facets which is safely performed before any laminectomy. Furthermore, especially on the concave side at the thoracic level, this maneuver is done with a Capener osteotome allowing easy exposure of the ligamentum flavum and its removal. We then gingerly remove the portion of the spinous process of the cephalad vertebra which overhangs the intervertebral space. Excision of the ligamentum flavum with a sharp knife risks inadvertent durotomy. It is advisable to use a rongeur to open the peridural space by resecting a small portion of the ligamentum flavum at the midline. The protection offered by the epidural fat at this level makes this maneuver relatively safe. We then introduce a Penfield 4 to verify the absence of attached dura to bone. Protected by the Penfield 4, we incise the ligamentum flavum sufficiently to allow the introduction of a Kerrison which finishes the excision of the ligamentum flavum. We continue to remove the superior border of the lamina and continue this excision as far lateral as possible in order to place the hook easily. Thus, the opening space must correspond to the width of the laminar hook. However, we recommend a slightly wider opening in case of epidural bleeding which can be freely evacuated to prevent a compressive epidural hematoma. On the other hand, an opening too generous leaves the potential that a hook would not be held sturdy to the rod. This turns out to be potentially dangerous and also risks weakening the mechanical resistance of the hooks. Introducing supralaminar hooks must be done without any force. The movement, is perhaps, comparable to a key which turns in a lock: the hook is introduced with the lamina perpendicular to the particular space, the end of the lamina hook itself is to be at the level of the superior border of the inferior lamina. The fundamental motion is a gentle rotation which brings the lamina hook under the lamina border. In certain cases, the deformation of the vertebral gutter may be so much as to make it difficult to use a hook holder. In these cases, it is very nice to use a Halsted pliers or any other comparable instrument to manipulate the hook. Moreover, this automatically decreases the risk of applying excessive force during this motion. This is particularly true during the utilization of supralaminar hooks at the most proximal thoracic level. The choice of the supralaminar hook depends on the role required of the hook. The supralaminar hook in the inferior aspect of the construct is acting in compression and will be pushed toward the canal during the rotation of the rod: therefore a « thoracic » small blade hook should be chosen which is designed to prevent excessive penetration into the canal. The inferior supralaminar hook of the two opposite distracting hooks on the concave side exerts a strong dorsal traction on the spinal segment in order to restore the thoracic kyphosis. It is, therefore, preferable to choose a laminar hook as large as possible despite the fact that this hook enters the canal at the thoracic level. Keep in mind that this hook will move away from the dura at the time of rotation of the rod

THE PEDICLE HOOKS

Placement of pedicle hooks in a primary surgery requires removal of the inferior border of the inferior articular process. There is a very noticeable change of the curve of the inferior border of the lamina which marks the internal border of the joint. With a 1/4 inch osteotome we first make a longitudinal cut along the medial border of the inferior articular process, then another cut parallel to the transverse axis of the vertebral body. This cut must be complete or the hook may migrate to occupy an infralaminar position. The osteotomy must be done through a controlled effort so that the superior articular process of the inferior vertebra is not entered. The pedicle finder is used to localize the pedicle by sliding it into the facet joint without undue constraint. We identify the pedicle finder instrument carefully because often an inexperienced surgeon may confuse the pedicle finder with the rod pusher which has a wider potentially dangerous blade if it is used as a pedicle finder. We then insert the pedicle hook which may be manipulated with the implant holder. For the entry of the hook it is held in a gently angled position in comparison to the articular space. With gentle wrist flexion of the surgeon, the hook penetrates the articular space which is more or less parallel to the general inclination of the vertebral body. This entire maneuver must be done without force to avoid penetrating the superior articular process of the joint. If the articular space is very tight, use a hook holder with a hook pusher to impact the hook on the pedicle. Sometimes it may be difficult to place a pedicle hook in the high dorsal region because of the prominent base of the transverse process inferior to the particular articular space. In this case the transverse process may be partially removed with a bone cutter to ease the placement of the pedicle hook.

THE SUPERIOR LIMIT OF THE CONSTRUCT

For reasons of stability, we prefer to end the construct with a bilateral superior claw. Up to T4 we use a pedicle-lamina claw on the same vertebra. To prepare for the placement of the transverse hook (which is in fact a large lumbar lamina hook) we use a transverse process elevator to slide under the particular transverse process. Once the elevator is positioned at the base of the transverse process we apply traction to ensure that it is actually under the transverse process. Superior to T4, we prefer a pedicle-lamina claw over two adjacent vertebra. The left and right supralaminar thoracic hooks adapt to the anatomy specific to this region. During the opening of the ligamentum flavum, it is particularly important to avoid the epidural venous plexuses which are abundant at this level. For this reason, it is often desirable to finish the placement of the hooks with this hook in order to prevent prolonged difficulty with the often excessive bleeding. The placement of the supralaminar thoracic hooks can be realized without resection of the ligamentum flavum by gently detaching it from the superior border of the lamina with a small Penfield. It is advisable to check the stability of these supralaminar hooks because their small blade may be insufficient in more kyphotic situations. In these cases a reduced blade hook should be used.

EXCISION OF JOINTS AND GRAFT

The next part of the operation is to remove the facet joints. This is a fundamental procedure because always remember that the goal is to obtain a solid fusion and hence a painless spine. All the residual joint capsules are carefully excised. The copious cancellous bone graft is applied to the fresh articular surfaces. The packing of bone graft into the joints also acts to decrease the amount of bleeding. At the lumbar level, the articular surfaces are removed by a rongeur or a gouge. At the apical lumbar level, the removal of facet joint may be total.

CONTOURING OF THE ROD

The contouring procedure is fundamental. It is guided by the vertebral profile that we want to obtain. It is incorrect to contour the rod solely to the coronal curve to facilitate rod introduction. The fundamental action in the reduction is the rotation maneuver. It is a guarantee of harmonious correction with corrective forces acting over the whole instrumented area of the spine together at the same moment. This is fundamental different from a local distraction on a short segment between two hooks which we should always avoid because of the neurological risks. The procedure works for us in order to obtain the goal of re-establishing the vertebral column balance. While contouring the rod, constantly note the axis of the rod, in order to bend in plane which is desired. Realize that the CD Horizon rod is slightly decreased in its strength compared to the classic CD rod (about 15%), and, therefore, it is necessary to bend the rod a little more than usual to account for the slight loss of bend which will occur during the rotation maneuver. This slight loss of contour is, nonetheless, also is a factor in preventing any excessive correction and its inherent neurologic risks.

PLACEMENT OF THE CONCAVE ROD

The concave rod is the first to introduce for the correction of a thoracic curvature as the automatic distraction which will occur during the rotation of rod acts in favor of the restoration of the thoracic kyphosis. For the lumbar curve, based on the same principle, the convex rod is used first to reestablish the lumbar lordosis. The introduction of the concave rod is greatly facilitated by the posteriorly opened hooks which also allow for a change of plan intraoperatively, such as beginning the introduction of the rod at a level which seems most appropriate to the correction sequence. The upper and lower most hooks can be used as closed hooks by placing the break off set screw (plug) in the hook prior to rod placement, which would help stabilize the rod during manipulation of the rod, or they can be used as open hooks if the specific local anatomic considerations exist. The correction of the spinal alignment begins with longitudinal traction during the procedure (Cotrel table), followed by placement of the concave rod with a sequential reduction of the spine through the rotation maneuver. A question often arises regarding the need to immobilize the hooks with hook holders during the rotation maneuver. In our experience the use of multiple hook holders during rotation of the rod is cumbersome and not necessary. Most importantly, manipulation of the rod must be done with great gentleness. Any sudden movement risks injury to the intradural canal from penetration of a hook. In the standard technique, the placement of the rod starts at the superior thoracic level. The rod enters first the throat opening of the pedicle hook followed by the complimentary hook of the superior claw. It is not necessary to hold the hook with a hook holder. It is helpful to control the position of the superior pedicle hook with a hook holder to prevent its dislodgment from its appropriate resting position and to avoid the threat of the pedicle hook injuring the dura especially during the rotation maneuver. It is the systematic utilization of the pedicle-transverse process claw or the pedicle-lamina claw at this level that prevents the medial translation of the superior pedicle hook. The break-off set screw at this level are introduced « free hand » with a plug holder. To avoid any cross-threading it is advisable to turn a quarter of a turn counterclockwise to ensure the thread of the plug is lined up with the thread of the implant. The set-screw should be slightly tightened in order to immobilize the two hooks of the superior claw on the rod. Then we proceed to place the rod in the most distal hooks. During this process we avoid placing any pressure on the intermediate hooks. We then introduce sequentially the rod into the intermediate hooks. This action is the first step in correction of the curvature. This type of segmental translation was already done before with the classic CDI during the introduction of the contoured rod into the intermediate hooks. With the new CD Horizon, this sequential correction is now realized in a controlled fashion. Three ancillary instruments are used to accomplish this goal. The first and mostly used is the translator (or approximator, slightly different instrument distributed in the US) which realizes segmental translation between hook and rod. The second is the « rocker » instrument in cases where there is a slight height difference between the rod and the hook especially at the distal limit of the construct on the side of the two opposite hooks. The third is the plug introducer, especially useful for aligning rod and hook in the sagittal plane. These three instruments allow also for a precise placement of the break-off set screw (plug). The quality of placement of this plug ultimately impacts the overall stability of the construct. Suboptimal placement of a plug translates into micromotion of the rod which may decrease the stability of the construct. Application of the translator: The implant holder part of the translator is aligned with the lateral edge of the hook. Once the hook is solidly in place through the implant holder of the translator, the central pusher part of the translator is to be fixed to the implant holder. This central pusher which is applied on the rod is fixed to the hook holder part through a ring opening. This connection should be tightened slightly so that it is not in the way of the translation movement. Once the translation toward the throat of the hook is realized the central pusher is screwed toward the bottom of hook. This assures the complete introduction of the rod to the bottom of the hook (fig. 8c). The plug driver is then introduced through the central opening of the translator to contact and tighten the plug. This is a simple step which obviates the need for visual control. The plug driver is turned a quarter of a turn counterclockwise to ensure the thread of the plug is lined up with the thread of the implant, then the plug driver is turned clockwise a number of turns. Tactile feel of a tight hold is necessary, yet allow a little mobility for the rotation maneuver. (fig 8d)

figure 8a

figure 8b

figure 8c

figure 8d

Fig. 8A,B,C,D: surgical technique: the translator

This maneuver is very simple and relatively quick. In contrast, when additional force is needed to seat the rod into the hook, we use a four-sided top guided plug introducer (fig 9a) with two corkscrew devices. This instrument is very powerful and overcomes certain shortcomings of the contour of the rod. The prongs of the plug introducer rest on the corresponding marks of the hook (fig. 9b). The two corkscrew devices are attached to the flanges of the plug inserter and slowly screwed on to the rod (fig 9c). This way a perfectly symmetrical introduction of the rod is realized. The plug implant is then placed into the cannulation of the inserter and provisionally secured with a plug driver. The introduction of the plug is the same as when using the translator. (fig. 9d) We recommend utilization of these two fundamental ancillary instruments for the direct plug tightening after manipulation with an implant holder, because without them there may be a false sense of security that a rod is fully seated at the bottom of the hook, when actually it is not. The plug introducer and corkscrew combination also usually prevents the plug from being placed obliquely in the threads of the hook. This is especially important for arthrodesis with pedicle screw fixations where there is no possibility of mobility at the bone-implant interface. In a case of cross-threading of the plug, the plug advancement is rapidly halted with a grinding noise and it becomes difficult to turn the plug driver. This plug should be immediately replaced.

figure 9a	figure 9b
figure 9c	figure 9d

Fig. 9A,B,C,D: surgical technique: the quadripode introducer

ROTATION OF THE ROD

Rotation of the concave rod is preceded by temporary fixation of the implant. This is accomplished by tightening the plugs into the threads of the hooks. Placement of C-ring instruments (compressors and distracters) will help maintain hook position during rotation of the rod. The curved « distractor » clamp should be used at the thoracic level and the « compressor » clamp at the lumbar level. This will assure a good hold of the hooks, which may have a tendency to move away from their positions during rod rotation. If one prefers to use the original C-rings, it is important to keep in mind the direction of rod rotation, so as to avoid having to untighten the C-ring plugs from a difficult position after rod rotation. Never forget to release all longitudinal traction just prior to the rotation maneuver. Use of the power grip and rod holders will aid in the rotation of the rod. The actual rotation maneuver is very slow and progressive to allow the viscoelastic properties to affect the reduction of the deformity. The progressive re-establishment of a better sagittal alignment is often expressed in a constant improvement of the somatosensory evoked potentials during the course of rod rotation. The evoked potentials may have been slightly altered during the previous placement of hooks (obstruction) and during segmental correction. Overall, the SSEP's usually improve and this beneficial effect may be interpreted as a strong argument in favor of the rotation correction maneuver. Always be cognizant that the pedicle hook has the potential of moving into the spinal canal to become a sublaminar hook and the inferior most infralaminar hook has the risk of pulling out posteriorly during the rod rotation maneuver. During rotation, we observe the inferior supralaminar thoracic hook to exert a very strong force on the lamina. This forceful pull upon derotation may actually push against the adjacent lamina. It is at this level that a crack may occur and may be a warning sign before complete disruption of the vertebral lamina. Once the rotation is complete, we strongly tighten the top of the plugs to maintain the correction. When the top of the plugs are in good position and correctly tightened, we must not see even the slightest amount of threads protruding posterior to the hooks. The hook placement must be verified, in particular, the inferior most supralaminar thoracic hook which may partially slip away from its desired sublaminar position. We then retighten the superior vertebral claw and the lumbar apical hooks. A question often arises regarding when to bend the rod in-situ. In our experience, it may be useful in lordosing the lumbar region if, after all the correction maneuvers are done, the lordosis remains insufficient. Then an accentuation of the bend by an in-situ maneuver may be necessary to perfect the correction. This procedure at the lumbar level includes a mechanical risk; this being a pull out of the inferior most infralaminar hook during the bend from excessive pressure. The risk of neurologic complication from in-situ bending in the lumbar region is small because the hooks would pull out in a direction away from the dura and there is relatively more space in the lumbar canal than in the thoracic canal. In contrast, at the thoracic level, the neurologic risk is very important as well as the mechanical risk. Hence, in-situ bending at the thoracic level is not recommended.

figure 10a

figure 10b

figure 10c

figure 10d

Fig. 10A,B: correction of a double major curve with the sequential CD Horizon technique: segmental translation followed by rotation of the rod

PLACEMENT OF THE CONVEX ROD

This rod is to augment the stability of the construct. It helps maintain the correction by stabilizing the construct. Concerning the placement of the rod, there is no particular difference to the opposite side. The advantages of the smooth rod are obvious compared to the classic CDI rod: the advancement is clearly very easy by the decreased friction of the smooth rod. The placement of the rod is facilitated through the top-loading by the posteriorly directed opening and closing of all the implants.

VERIFICATION OF THE HOOKS

The goal of this control is to assure that all hooks are perfectly seated against the lamina. Especially, the most inferior infralaminar lumbar hook has to be checked: if the hook blade isn’t perfectly seated then one should gently push down on the distal tip of the rod to disengage the hook blade from the lamina. Otherwise any further compression would only result in a laminar fracture without any cephalad progression of the hook blade. All hooks are verified but once again, forceful distraction should be avoided.

PLACEMENT OF THE DTT

The device of transverse traction (DTT) is used in the direction of distraction between the two rods. This distraction improves the mechanical hold of the laminar hooks in a position as lateral under the lamina as possible. Nonetheless, too much distractive force could cause the pedicle hooks to slip lateral also. The placement of the CD Horizon DTT is clearly very easy and quicker than the classic CDI DTT. A long construct may require 2 or 3 DTTs. A newer form of the DTT may be used in which there are several models of different predetermined lengths. This newer DTT does not require much adjustment. To place it in adequate tension we use a distracter between the two rods which then permits the placement of the adaptable length DTT.

FINAL TIGHTENING AND BREAK OFF OF SCREW HEADS

During the maneuver of screw head (plug head) break off, the hook is immobilized with the use of a counter-torque device. This helps avoid excessive stress on the hooks during the screw break off. Sometimes the placement of a counter-torque device is impossible because of a DTT being too closely placed to a hook, thus not allowing enough room for the counter-torque device. For this reason, it is often preferable to break the top of the screw before placing the DTT. If necessary, the screws can always be removed to allow for a modification of the construct prior to breaking off the screw heads. If this is done, it is important to exchange the screw for a new one to assure its strength. Concerning the problems with revision of the construct during the primary surgery, remember that the CD Horizon allows this maneuver very easily. On the other hand, a later modification (second surgery), for example an extension of the construct, will often be fraught with challenges due to the presence of a large fusion mass surrounding the rods and hooks, similar to the classic CD system.

BONY AUGMENTATION

All accessible bony surfaces need to be decorticated and grafted. This is a fundamental part of the operation. Instead of removing the spinous processes, we decorticate them with a Capener gouge to augment the surface area for fusion. Furthermore, it is best not to remove the spinous processes especially in the thoracic region, to prevent the construct from lying subcutaneously.

Fermeture

Closure of the fascial and muscle layers is with multiple interrupted sutures. A subcutaneous drain is especially used if there is a subcutaneous tunnel used to obtain iliac crest bone graft; the drain is not placed to suction during the first 24 hours postoperatively to decrease the blood loss, then it may be placed to bulb suction thereafter. A subcuticular suture is placed and a compressive dressing is applied.

POSTOPERATIVE CARE

Patients should be out of bed and walking postoperatively as rapidly as possible. This is usually accomplished by the third postoperative day in our institution. Respiratory therapy should be used immediately and continued until recuperation is to a preoperative state. Similarly, the patient must learn to control their new posture in front of a mirror in order to re-educate their proprioceptive mechanisms. It has been noted that almost all patients have the sensation of being crooked postoperatively. Therefore, there is a tendency for patients to want to return to their preoperative position. The postures in front of a mirror are therefore, very helpful for the patient to be able to adapt to this new position in space. We observe a clinical re-equilibration, which for certain patients is difficult to adapt to for several weeks.

Conclusions

The description given here may give the impression that the surgical procedure is very complex. In fact, the technique is relatively simple and above all it is simplified in comparison to the classic CD system. Those who had the occasion to use the classic CDI remember its efficiency but also the multiple difficulties including: making the opening of a transverse process hook line up with a pedicle hook at the proximal claw, the advancement of the diamond point rod through the closed hooks, the high profile of the materials especially important in neuromuscular patients (fig. 11a,b,c,d), and the revision for a modification or an extension of the construct, etc. All of these issues are simplified in the CD Horizon system. In particular, the reduction of the profile of the material definitely decreases the discomfort caused by the material and augments the stability of the construct by increased direct contact with the posterior vertebral arches. The initial concern that the 15% decreased strength of the rod would decrease its ability to yield a reduction of the curve, has not been confirmed in our experience. On the contrary, the ancillary instruments allow for a controlled translation correction prior to the application of the rotation maneuver. This certainly diminishes the strain on the rod during the rotation maneuver. Other essential characteristics are the precise introduction of the fixation plugs with the ancillary introduction equipment and the break off screws which are posteriorly tightened. Lastly, even though this system is easily utilized, it is recommended to surgeons who only occasionally operate on adolescent scoliosis, to first assist a colleague who is comfortable in the use of this system.

figure 11a

figure 11b

figure 11c

figure 11d

Fig. 11 A,B,C,D: Scoliosis in a patient with congenital muscular dystrophy (community walker). Note the good correction of the sagittal imbalance