34th Annual Scientific Meeting proceedings

A radiographic image of a horse with impinging dorsal spinous processes (IDSPs) presents a clinical conundrum; Are the IDSPs clinically relevant? How should the case be managed?  If surgery is performed, with which technique? Even in people where advanced imaging of the spine and pelvis is readily available, 85% of lower back pain cases lack a clearly identifiable cause.¹  It is therefore no wonder that answers to questions relating to IDSPs in the horse are unclear and that there is no consensus: A recent survey of equine specialists revealed diverse treatment strategies for horses with IDSPs influenced by region and area of specialisation.²

Clinical signs potentially related to IDSPs can include back pain or stiffness, muscle wastage, an unwillingness to go forwards, bucking, pain on saddling and other gait abnormalities such as an abnormal canter, poor hindlimb impulsion and difficulty in transitions.  Diagnostics should always incorporate careful palpation, a complete dynamic assessment for lameness, assessment of saddle fit and radiography of the DSPs.  The main location of IDSPs is between T14 and L2, with a wide variation in appearance, including in shape, interspinous width, margination and the presence of bony modelling.  Their aetiology is unclear, although a study by Pressanto et al.³ supports a developmental aetiology.  IDSPs occur in around a third of normal horses, with thoroughbreds over-represented,^4,5 and there is conflicting evidence regarding the correlation in degree of severity on radiography and scintigraphy with clinical significance.⁶ It has been hypothesised that this lack of correlation may be due to inconsistent upregulation of nociceptive fibres in the interspinous ligament of horses with IDSPs.⁷ An association has been identified between increasing severity of IDSPs and the presence of osteoarthritis of the thoracolumbar articular process joints.⁵

Depending on clinical signs, availability and budget, further diagnostics that should be considered to identify concurrent pathology include further radiography and ultrasonography of the spine, gamma scintigraphy of the back, hindlimbs and pelvis, serum biochemistry to assess creatinine kinase and aminotransferase at rest and after exercise, muscle biopsy, and computer tomography of the neck or lumbosacral and pelvic region.  Diagnostic anaesthesia of the thoracolumbar region can be useful,⁸ but it is important to note that it is non-specific, can improve dorsoventral motion in clinically normal horses,⁹ and in some cases is not possible due to safety concerns. Potentially clinically relevant findings include lameness, vertebral body spondylosis, thoracolumbar articular process joint pathology, cervical arthropathy, pathology of the sacral or lumbar regions and sacroiliac joint pathology.  de Souza et al.¹⁰ demonstrated a poorer prognosis following surgery of IDSPs when other orthopaedic issues were concurrently identified, highlighting the importance of their identification and caution when considering surgery in such cases.

Once the decision has been made that IDSPs may be contributing to the clinical picture, treatment is often medical and can include systemic non-steroidal anti-inflammatories, medication of the region of the IDSPs with corticosteroids, shockwave, neuromuscular electrical stimulation (NMES), dynamic mobilisation and controlled, initially non-ridden exercise.  Both NMES and dynamic mobilisation have been shown to increase the cross-sectional area of the multifidus muscle and so could potentially improve spinal stabilisation.¹¹ Saddle fit, training and the rider should also be considered.  There are currently no well-designed randomised trials comparing medical and surgical treatment of IDSPs.  If medical management of IDSPs has failed and other pathology has been ruled out as far as possible, various surgical options are available, including interspinous ligament (ISL) desmotomy, total ostectomy, subtotal ostectomy (which can be cranial or caudal), and minimally invasive subtotal ostectomy, all of which are now most often performed under standing sedation.^{12,13,14,15,16,10} The reported excellent surgical success for return to work following all these techniques (ranging between 79 and 86%)^{12,13,14,15,16,10} may fuel one’s enthusiasm to operate, as may the low complications rates reported.  Complications, which are rare, and in most cases not clinically significant, include infection, neurogenic atrophy and swelling at the surgical site.  Interestingly in a recent ex vivo study neither interspinous ligament desmotomy nor cranial wedge ostectomy resulted in an increased range of motion during flexion, extension or lateral bending but both procedures influenced the rotational component of thoracolumar mobility.¹⁷ Following ISL desmotomy multifidus muscle injury was evident at all desmotomy sites and there was no increase in the interspinous width.¹⁷

As our ability to utilise advanced imaging of the equine spine and pelvis becomes more readily available and our understanding of the clinical relevance of various pathologies increases the answer to the question “Spinous Processes – to operate or not to opearte” may become clearer.  Surgical treatment of IDSPs is successful in the majority of cases with minimal complications.

References

1. Refshauge, K.M., Maher, C.G.  (2006) Low back pain investigations and prognosis: a review.  British Journal of Sports Medicine 40(6):494–498.
2. Treß D, Lischer C, Merle R, Ehrle A. (2024) International survey of equine orthopaedic specialists reveals diverse treatment strategies for horses with overriding spinous processes. Veterinary Record, e3899.
3. Pressanto, M.C., Pepe, M., Coomer, R.P.C., Pilati, N. & Beccati, F. (2023) Radiographic abnormalities of the thoracolumbar spinous processes do not differ between yearling and trained Thoroughbred horses without perceived back pain. Journal of the American Veterinary Medical Association, 261(6), 844–851.
4. Jeffcott, L.B. (1980) Disorders of the thoracolumbar spine of the horse - a survey of 443 cases. Equine Veterinary Journal, 12, 197–210.
5. Zimmerman, M., Dyson, S. & Murray, R. (2012) Close, impinging and overriding spinous processes in the thoracolumbar spine: the relationship between radiological and scintigraphic findings and clinical signs. Equine Veterinary Journal, 44(2), 178–184.
6. Looijen, M.G.P. & Morgan, R.E. (2024) Should radiographs of the thoracolumbar spine remain part of the pre- purchase examination? Equine Veterinary Education, 36, 438–448.
7. Ehrle, A., Ressel, L., Ricci, E., Merle, R. & Singer, E.R. (2019) Histological examination of the interspinous ligament in horses with overriding spinous processes. Veterinary Journal, 244, 69–74.
8. Brown, K.A., Davidson, E.J., Ortved, K. (2020) Long-­term outcome and effect of diagnostic analgesia in horses undergoing interspinous ligament desmotomy for overriding dorsal spinous processes. Veterinary Surgery, 49(3):590–­9.
9. Derham, A. M., Schumacher J., O’ Leary J.M., Kelly G and Hahn C.N. (2021) Implications of the neuroanatomy of the  equine thoracolumbar vertebral column with regional anaesthesia and complications following desmotomy of the interspinous ligament. Equine Veterinary Journal, 53:649–655.
10. de Souza, T.C., Crowe, O.M., Bowles, D., Poore, L.A. and Suthers, J.M. (2022) Minimally invasive cranial ostectomy for the treatment of impinging dorsal spinous processes in 102 standing horses. Veterinary Surgery, 51(S1):O60‐O68
11. Lucas, R.G., Rodríguez-Hurtado, I., Álvarez, C.T. and Ortiz, G. (2022) Effectiveness of neuromuscular electrical stimulation and dynamic mobilization exercises on equine multifidus muscle cross-sectional area. Journal of Equine Veterinary Science 113, 103934
12. Coomer, R.P.C., McKane, S.A., Smith, N., Vandeweerd, J.-M.E., 2012. A controlled study evaluating a novel surgical treatment for kissing spines in standing sedated horses. Veterinary Surgery 41, 890–897.
13. Derham, A.M., O’Leary, J.M., Connolly, S.E., Schumacher, J. and Kelly, G. (2019) Performance comparison of 159 Thoroughbred racehorses and matched cohorts before and after desmotomy of the interspinous ligament. The Veterinary Journal, 249, 16–23
14. Walmsley, J.P., Pettersson, H., Winberg, F. and McEvoy, F. (2002) Impingement of the dorsal spinous processes in two hundred and fifteen horses: case selection, surgical technique and results. Equine Veterinary Journal 34, 23–28.
15. Jacklin, B.D., Minshall, G.J., Wright, I.M., 2014. A new technique for subtotal (cranial wedge) ostectomy in the treatment of impinging/overriding spinous processes: description of technique and outcome of 25 cases. Equine Veterinary Journal 46, 339–344.
16. Connaughton, M.T., MacDonald, E.J., Ireland, J.L., Rocchigiani, G. and Stack, J.D. (2025) Experimental comparison of caudal wedge ostectomy to cranial wedge ostectomy for surgical treatment of overriding/impinging spinous processes in horses. Equine Veterinary Journal, https://doi.org/10.1111/evj.14498
17. Baudisch N., Singer, E., Jensen, K.C., Eichler, F., Meyer, H.J, Lischer, C. and Ehrle, A. (2025) Influence of surgical intervention at the level of the dorsal spinous processes on the biomechanics of the equine thoracolumbar spine. Equine Veterinary Journal 57, 2, 409-501.