24.1 The Restoration and/or Preservation of Sagittal Balance

The restoration and/or preservation of a normal or nearly normal sagittal balance should be considered with nearly all operative procedures in the lumbosacral region (Fig. 24.1). For example, decompression (Fig. 24.2) and uninstrumented fusion (Fig. 24.3), as well as instrumented lumbar–sacral fusion procedures, can result in loss of lordosis and a loss of sagittal balance. Instrumented fusions usually cause a loss of lordosis via dorsal distraction, which results in a bending moment–derived flexion (kyphosis; Fig. 24.4). This flattening of the back is associated with two fundamental problems: (1) a characteristic pain syndrome (flat-back syndrome) and (2) the application of adverse forces and stresses to the spine, both in the region of the fusion and at adjacent segments, as a result of a nonphysiologic alignment. These iatrogenic adverse structural consequences can result in an augmented risk for construct failure and accelerated end-fusion degenerative changes.

24.1.1 Flat-Back Syndrome

The flat-back syndrome is associated with a fixed forward inclination of the trunk and an inability to stand erect. The knees are usually flexed in order to facilitate forward vision and an erect posture. Back pain is the most prominent symptom and is typically localized to the upper back. Patients usually describe the pain as worsening during exertion or assumption of the upright posture (standing). This fatigue-induced pain is secondary to efforts to hyperextend the thoracic and cervical spine in order to stand erect. Most patients report a worsening of posture and associated upper back pain and cervical pain as the day progresses. Many patients also report lower back pain as well as pain and tightness in the quadriceps and hamstring region (ventral and dorsal thigh). The affected patient is unable to assume an erect posture and acquire forward vision without flexing the knees and so leans backward “from the knees.” This constellation of symptoms and signs is related to an inability to extend a pelvis that is fixed to the low lumbar spine in a flexed posture. Therefore, the only methods by which pelvic extension can be achieved are by knee flexion and/or by excessive upper thoracic or cervical extension—hence, the prominence of upper thoracic and cervical pain (Fig. 24.5).

24.1.2 Deformity Prevention and Correction Strategies

A facile and savvy surgeon can indeed correct deformity and achieve sagittal balance in nearly all cases. In some cases, however, excessive surgery and so excessive risk may be required to achieve this. The surgeon, therefore, must question “how much” to do from a surgical perspective and “how much” risk to take in order to achieve the desired correction. One must carefully consider the preoperative symptoms and the patient’s expectations. Regarding the latter, the patient’s expectations must be aligned with the surgeon’s goals before surgery. Therefore, a thorough, informed preoperative decision-making process is essential.

Preservation of the normal lordotic posture is critical. When lost in a prior fusion procedure, an osteotomy may be considered as a treatment option in the appropriately symptomatic patient. The iatrogenc surgical loss of lordosis can usually be prevented. This can be accomplished, in part, by intraoperative positioning. For example, the alignment and fusion of the spine in a more physiologic position (sagittally balanced) are assisted by hip extension (and the avoidance of intraoperative hip flexion) during surgery (Fig. 24.6).

Wedge osteotomy is an extensive procedure that should be selectively used to correct fixed deformities in patients with intractable pain or neurologic deficit. With this procedure, the goal is the achievement of a normal or nearly normal sagittal alignment and an associated reduction of pain. There are fundamentally two surgical approaches that can be employed to achieve extension of a lumbar spine with wedge osteotomy. These are depicted in Fig. 24.7 and Fig. 24.8. Each uses a different axis of rotation for deformity correction.

24.1.3 Iatrogenic Adverse Structural Outcomes

An iatrogenic loss of sagittal balance and alignment increases the chance of construct failure, as well as the incidence of accelerated end-fusion degenerative changes. The creation of an abnormally aligned spine alters the moment arm through which forces are applied to the spine. This results in an increased bending moment and stress application, with an increased chance for construct failure (Fig. 24.9a).

End-fusion degenerative changes are accelerated by repetitive eccentric loading of a disc and by loading of a disc in a nonphysiologic or unnatural manner (Fig. 24.9b), as would result following the creation of a loss of normal lumbar lordosis (see Fig. 24.2 through Fig. 24.5 and Fig. 24.9c).

24.2 The Restoration and/or Preservation of Neurologic Function

The importance of restoring and preserving neurologic function cannot be overstated. Occasionally, however, neural injury is a component of a successful lumbosacral–pelvic operation, particularly when sacral resection for tumor is the goal of surgery. ^{2 ,​ 3} In general, the preservation of both the S1 and S2 nerve roots will usually preserve lower extremity motor and sensory function, as well as bowel and bladder function. The latter is sacrificed following total sacrectomy (Fig. 24.10). ^{2 ,​ 4} Ultimately, the preservation of S3 is associated with the preservation of bowel and bladder function. ³

Such operations are associated with infectious and other complications. The incidence of infection is increased when spinal instrumentation is employed, the surgery is a redo operation, the albumin is less than 3.0 g/dL, and the operation time is excessive. ⁵

Obviously, resection of the coccyx is associated with minimal risk for neurologic injury. It may uncommonly be indicated in patients with coccygeal pain (coccydynia). ⁶ Sacral resection, including high sacral resection, can be enhanced by using thread-wire saw sacral amputation. ⁷

24.3 The Augmentation of Lumbosacral Stability

The acquisition of acute (short-term or early) spinal stability is best achieved by surgery that is minimally structurally destructive. A thorough knowledge of regional anatomy ^{8 – 10} and biomechanics is therefore imperative, ^{11 – 13} including the biomechanical considerations that are associated with the axis about which the lumbosacral region rotates (flexes and extends) in the sagittal plane. This has been defined by McCord and colleagues and is termed the lumbosacral pivot point (Fig. 24.11). ^{12 ,​ 13} It essentially is located at the level of the dorsal annulus fibrosis of the L5–S1 motion segment. The lumbosacral pivot point is more than an axis about which the lumbosacral region rotates in the sagittal plane. It represents a point that must be exceeded (ventrally or caudally) by a fixator (e.g., screw or hook) in order to optimally stabilize the spine from a sagittal plane rotation perspective. Dorsal implants that provide a moment arm that extends ventral to this point can effectively resist rotation (Fig. 24.12a). Appropriately placed S1 screws can provide this. The ilium can obstruct such appropriate placement. Kaptanoglu et al have devised a strategy to overcome these limitations. ¹⁴ An implant that extends caudal to this point can also provide a biomechanical advantage (Fig. 24.12b, c). A combination strategy presents a still greater advantage (see Chapter 28 and Fig. 24.12c). Alegre et al demonstrated that with long constructs extended to the sacrum, S1 screw bending moment was decreased in flexion–extension when the long construct was extended either to the ilium or to S2 via an S2 sacral screw. They also demonstrated no advantage of the bolt over the S2 screw. They did, however, also observe that adding ventral axial load-bearing support significantly decreased the bending moment on the S1 screw. ¹⁵ Cunningham et al have confirmed these findings. ¹⁶ Of note, the L5–S1 interbody fusion support is essentially the equivalent of passing a dorsal screw ventral to the lumbosacral pivot point. This has been confirmed clinically. ¹⁷ Hence, screw length is important. Shorter iliac screws provide suboptimal fixation potential. Augmentation of short screws with bone cement somewhat compensates for their short length. ¹⁸ Iliac bolt fixation may provide slightly shorter ventral fixation, and the security and ease of fixation may outweigh its disadvantages in some cirmumstances. ¹⁹

These concepts are of particular clinical relevance because of the inability to obtain fixation points with long moment arms in the sacral region (as can be obtained with more rostral constructs). Of note is the fact that S2 dorsal neural foramina hooks and S1 sublaminar wires or cables provide an additional excellent fixation point ( Fig. 24.13 ). ²⁰ If a total sacrectomy is performed, other aggressive strategies must be used to achieve spinal stability (see the following). ^{2 ,​ 21}

Clinically appropriate fixation is also enhanced by the use of multiple fixation points and the triangulation effect. These biomechanical advantages can be enhanced with instrumentation adjuncts such as the Chopin and Tacoma devices ( Fig. 24.14 ). Current instrumentation strategies, such as ilial fixation, obviate the need for such adjuncts, though.

24.3.1 Ilial Fixation

Ilial fixation is a useful method of augmenting lumbosacral fixation (or, uncommonly, iliosacral fusion ²² ). It also is used when sacrectomy has been performed for tumor (Fig. 24.15 a and b). Dorsal approaches to tumor resection facilitate the decompression (i.e., tumor removal) and instrumentation and fusion through the same approach. ²³ Often, however, the ilium is thin, thus precluding long intra-ilial rod (e.g., Galveston and slingshot techniques) or screw placement. Bicortical ilial fixation ²⁴ and other forms of ilial screw fixation are an alternative to intra-ilial fixation (Fig. 24.15c, d). ^{2 ,​ 25 – 31} Care must be taken with all of the aforementioned strategies so that the screws or rods project through the bone of the ilium ventral to the lumbosacral pivot point. This is necessary so that an adequate moment arm length is achieved to prevent sacral–pelvic flexion. Alternative techniques have been employed. ^{10 ,​ 32 ,​ 33} The combination of ilial and S1 screws provides superior fixation for lumbosacral fusion. This has been clinically demonstrated. ³⁴

Sacroiliac joint pain is a controversial entity. Its diagnosis and management are based on less than truly objective data. Some have suggested arthrodesis of the sacroiliac joint as a viable management strategy. ^{35 – 38}

Related posts:

25 Regional Nuances 21 Ventral Subaxial Spine Constructs 23 Interbody Constructs 20 Craniocervical and Upper Cervical Constructs 22 Dorsal Subaxial Spine Constructs 11 Destabilizing Effects of Spine Surgery

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