Spinal deformities and the “Goel instability concept”

Spinal deformity is conventionally defined by abnormal alignment. Kyphosis, short neck, torticollis, scoliosis, spondylolisthesis, and sagittal imbalance are diagnosed and quantified using radiological parameters, and treatment is generally directed toward correction of the deformity, decompression of neural structures, and stabilization when necessary. Although this approach has produced increasingly sophisticated reconstructive techniques, it raises a fundamental question: Is the deformity itself the primary pathology, or is it the consequence of an underlying instability?

The Goel Instability Concept proposes a different way of understanding spinal deformity. Its central premise is that local, and in some circumstances remote, spinal instability may be the primary pathological event, with deformity developing as a consequence. If this premise is correct, identifying the unstable spinal segments and stabilizing them may address the fundamental pathology. The concept therefore shifts attention from correcting the radiological appearance of the spine to identifying and treating the pathological process that produced it.

THE FACET JOINT: A CENTRAL STABILIZING STRUCTURE

A particularly important component of the Goel concept is the role attributed to the facet joints. The muscles responsible for maintaining the upright bipedal posture are predominantly located in the extensor compartment, and their actions are closely related to the posterior elements and facet articulations, which serve as important stabilizing structures and fulcrums for spinal movement.[1,2] According to the concept, weakness or dysfunction of these stabilizing mechanisms due to altered use, disuse, or injury may contribute to facet instability, listhesis of the facet of a rostral vertebra over a caudal vertebra at one or more spinal levels, or “vertical spinal instability.”[3] In its early stages, such instability may be subtle and difficult to identify, even with dynamic imaging. Progressive facet-joint vertical spinal instability is proposed as a contributor to the changes traditionally grouped under “spinal degeneration.”

Disc-space reduction, osteophyte formation, ligamentous buckling, and abnormal bone or cyst formation may, from this perspective, represent adaptive responses attempting to restrict abnormal movement and protect neural structures. These spinal alterations, which are generally considered pathological, are naturally protective or adaptive events, indicate segmental spinal instability, and are potentially or manifestly reversible after spinal stabilization. Progressive instability may subsequently contribute to deformity and alteration of global sagittal balance.

Thus, if deformity is partly a manifestation of pathological movement, eliminating that movement may be more fundamental than simply correcting the resulting geometry. Stabilization could potentially permit restoration toward a more physiological configuration and relieve secondary neural compromise.

CERVICAL DEFORMITY: A PROVOCATIVE APPLICATION

Degenerative cervical kyphosis is traditionally attributed to disc degeneration, vertebral collapse, posterior structural failure, or combinations of these processes. Depending on severity and rigidity, treatment may involve anterior, posterior, or combined reconstruction, often with neural decompression.

The instability concept offers another interpretation. In selected patients, cervical kyphosis may be produced or maintained by instability at specific motion segments. Atlantoaxial instability either in isolation or in collaboration with subaxial spinal instability may form the basis of spinal “degeneration” in general and spinal deformities like kyphosis in particular.[4] If spinal instability at the level/s of kyphosis or in a remote region like the atlantoaxial joint and the related abnormal movements contribute to both deformity and neural dysfunction, identification of the unstable spinal segments on the basis of clinical and radiological guides and stabilization of the responsible segments may address the underlying mechanical problem while providing indirect neural decompression.

Goel’s reported experience with degenerative cervical kyphosis treated by fixation without conventional decompression is particularly significant in this context.[4] It challenges the assumption that demonstrated neural compression necessarily requires direct removal of the compressive structure. More fundamentally, it changes the therapeutic target–from the compressed neural structure to the unstable spinal segment.

The concept therefore suggests that neurological compromise may be secondary to instability and that stabilization may be sufficient to improve both the mechanical and neurological components of the disease.

THE CRANIOVERTEBRAL JUNCTION: THE CLEAREST MODEL

The craniovertebral junction provides perhaps the strongest clinical expression of the Goel philosophy. Conditions such as basilar invagination and atlantoaxial dislocation have traditionally been understood primarily as abnormalities of bone relationships, often accompanied by neural compression. Treatment has consequently included decompression. Goel proposed for the first time in the literature that, in these patients, the fundamental abnormality is atlantoaxial instability.[5,6] Abnormal facet relationships, including vertical instability, may progressively alter the relationship between the atlas, axis, and skull base. The resulting changes can include upward migration of the odontoid and distortion or compression of neural structures.

In chronic or longstanding atlantoaxial instability, classical radiological parameters that identify atlantoaxial instability like abnormal increase in the atlantodental interval or neural or dural compression opposite the odontoid process, may not be present. Instability in such cases can be identified by facet alignment in lateral profile imaging or by dynamic head rotation or head tilting images. Such instability has been identified as central or axial atlantoaxial instability (CAAD).[7,8,9] A high degree of clinical understanding and the classical telltale clinical and radiological evidence ultimately lead to the diagnosis of instability. The therapeutic implication is substantial: if instability is primary, stabilization should be central to treatment. It challenges the assumption that the radiological deformity must always be directly corrected or that neural decompression must necessarily be performed.

The concept has also stimulated reconsideration of the relationship between craniovertebral instability, Chiari formation, basilar invagination, syringomyelia, and structural variations such as Klippel-Feil anomalies, bifid arches of the atlas or axis, and os-odontoideum, which have traditionally been regarded as abnormalities related to embryological dysgenesis.[10,11,12,13] The hypothesis that they represent adaptive or protective responses to chronic instability shifts attention toward causation rather than simply anatomical association.

Similarly, short neck, altered head and neck proportions, torticollis, and even remote spinal deformities such as dorsal kyphoscoliosis have been proposed as possible consequences of chronic craniovertebral instability or CAAD.[14,15,16] If such relationships are confirmed, treatment of the primary instability rather than secondary deformity could represent a more rational strategy.

EXTENSION TO DEGENERATIVE SPINAL DEFORMITY

Degenerative scoliosis is one of the several manifestations of chronic and progressive vertical spinal instability that may be asymmetric instability. Unequal loading may contribute to asymmetric disc and facet degeneration, lateral translation, rotation, and eventually a structural curve. In selected patients, identifying and stabilizing the pathological segments may therefore be more important than correcting every radiological component of the deformity.

This possibility is particularly relevant in an era of increasingly extensive reconstructive surgery. Long-segment fixation and complex osteotomies can produce impressive radiological correction but are substantial procedures with significant biological and mechanical consequences. If deformity is driven predominantly by a limited number of unstable segments, treatment directed at those segments could potentially achieve the primary mechanical objective with less extensive intervention.

ADULT IDIOPATHIC DE NOVO LUMBAR SCOLIOSIS

Adult idiopathic de novo lumbar scoliosis provides another interesting setting in which to consider the instability hypothesis. Traditionally, the development of a new lumbar curve in adulthood has been attributed to asymmetric disc and facet degeneration, loss of disc height, vertebral rotation and translation, and progressive coronal and sagittal imbalance. The conventional surgical response is therefore often directed toward correcting the established deformity, restoring alignment, and decompressing neural structures when stenosis is present.

From the instability perspective, however, the degenerative curve may be viewed as the consequence of asymmetric segmental instability rather than simply as a structural curve requiring extensive correction.

This raises the provocative possibility of treating selected patients with fixation alone, without attempting complete radiological correction or routine direct decompression. By stabilizing the presumed pathological segments, the abnormal movement may be eliminated, potentially allowing the spine to settle into a more physiological configuration while addressing the mechanical source of symptoms.[17] Such an approach could offer a substantially less invasive alternative to long-segment deformity correction in appropriately selected patients.

CHALLENGING THE DECOMPRESSION PARADIGM

Perhaps the most provocative aspect of the instability concept is its challenge to the traditional decompression paradigm.

The conventional model is straightforward: neural compression causes symptoms; therefore, the compressive structure should be removed. The instability model proposes an alternative sequence: instability produces abnormal motion, deformity, and secondary neural compromise; therefore, stabilization may address the primary pathology. This distinction has practical consequences. Decompression can itself further destabilize a motion segment when stabilizing structures are removed. The concept of “only fixation” in Goel’s surgical practice therefore challenges the assumption that decompression must automatically be the primary treatment whenever neural compression is demonstrated.

WHAT REMAINS UNANSWERED?

The instability hypothesis should not be interpreted to mean that every spinal deformity is caused by instability. Congenital deformities, neuromuscular scoliosis, rigid ankylosing disorders, severe vertebral destruction, trauma, and established fixed deformities have distinct mechanisms and may require reconstruction or an alternative form of treatment.

A CHANGE IN SURGICAL THINKING

The lasting importance of the Goel Instability Concept may therefore lie less in any individual surgical technique and more in the change in surgical thinking that it proposes.

The surgeon is encouraged to look beyond the deformity and ask why it developed. The relevant question is not simply, “How much deformity does this patient have?” but rather, “Why has this deformity developed, and is instability responsible?”

Not every kyphosis requires an osteotomy. Not every neural compression requires direct decompression. Not every radiological abnormality necessarily represents a primary structural disease.

Spinal deformity may sometimes be the final visible expression of a much earlier process—segmental instability. Recognizing that process could change how deformities are classified, investigated, and treated.

The proposition that “instability is the issue and stabilization is the treatment” therefore deserves consideration beyond the craniovertebral junction. It offers a potentially unifying framework extending from atlantoaxial dislocation and basilar invagination to cervical kyphosis, adult de novo lumbar scoliosis, degenerative spondylolisthesis, and selected degenerative spinal deformities.

Ultimately, the Goel concept represents a shift from alignment-centered surgery to instability-centered surgery. Rather than focusing exclusively on correcting what the spine has become, it asks the surgeon to understand why it became that way. These concepts provide an important and provocative framework for reconsidering the relationship between instability, deformity, and neural dysfunction.