Managing the Horse With Overriding Dorsal Spinous Process

Aric Adams | DVM, DACVS Equine Medical Center of Ocala

Overriding dorsal spinous processes (ODSP) can be a common source of back pain in the working horse. It seems that this has been a focus of poor performance for decades in Europe, and, more recently, it has also become a focal topic of poor performance in North America. Because it can cause issues that include training poorly, jump refusal, bucking, and resentment to saddling amongst other training issues, it has become the diagnosis of convenience in many cases. Given the difficulty in accurately diagnosing ODSP, it can be equally confusing when trying to evaluate the success of various treatments for this disorder. We will touch on the diagnosis of ODSP and then dive deeper into treatment options for these horses.

Diagnosis

The radiographic diagnosis of overriding dorsal spinous processes is more straightforward than interpreting what the findings mean. These days, most equine veterinarians possess digital radiography equipment that is fully capable of taking acceptable thoracolumbar radiographs to allow for evaluation of the dorsal spinous processes. Practitioners must be cautious when interpreting the interspinous space at the periphery of the radiograph due to divergent beam angle.¹ Some very large horses may require large overhead X-ray generators more commonly seen at referral hospitals to obtain good thoracolumbar radiographs, especially when attempting to evaluate the vertebral facets and vertebral bodies. Correlating radiographic changes in the dorsal spinous processes (DSPs) with training problems in horses is much more complex. Some horses have significant changes in the dorsal spinous processes, including overriding, bone proliferation, and bone resorption. Plus, they have no detectable clinical signs associated with these changes. Other horses may have only mild changes with seemingly severe clinical signs. Obviously, ODSP is only one of many potential problems that can cause back pain in horses. Poor saddle fit,² thoracolumbar vertebral facet osteoarthritis,³ sacroiliac osteoarthritis and pain,⁴ and multifidus and longissimus muscle injuries are other common sources of back pain that should be investigated. Identifying horses with back pain is very subjective. This becomes even more clear when attempts are made to make a grading system in which the assessment for back pain includes pain to digital pressure, poor development of epaxial muscles, warmness in the back region, swelling, resistance to lateral bending, bad attitude, and poor hind limb impulsion – which are all inherently subjective.⁵ Still, response to back palpation is probably the most commonly employed method to verify if a horse does indeed have back pain.

In the author’s experience, the horses that are being treated tend to be “fragile flowers” or more sensitive and responsive horses than the general population. These horses may also have needle aversions and be hyperresponsive to tendon and ligament palpation on distal limbs.

This makes response to back palpation particularly challenging. Blocking the interspinous spaces adjacent to the ODSPs has been advocated to help identify if it improves their back pain or their problems when being ridden⁶ as this helps when deciding if surgical intervention is more likely to improve them. However, one must also be aware that it has been shown that diagnostic infiltration of local anesthetic in normal horses does affect range of motion and dorsoventral flexion and extension⁷ which may complicate interpretation. Others also have used positive response to injections with corticosteroids adjacent to the ODSP to help verify them as the source of back pain.⁸The use of nuclear scintigraphy has been thought to be more specific for clinical ODSP than radiography alone and may aid in differentiating from sacroiliac disease or other causes of back pain.⁹ In this study, 582 horses were presented for poor performance and back pain.⁹ Only 46% of horses with radiographic ODSP and only 25% of horses with scintigraphic uptake showed signs of back pain, while 83% of horses with marked radiographic changes and moderate scintigraphic uptake showed signs of back pain.

Management Of Horses With Odsps

Horses with overriding dorsal spinous processes that have clinical back pain are “managed” because the various treatments that are provided are often ongoing throughout the horse’s career. Even after surgical intervention, some horses may still require additional treatments to help them to continue to perform and decrease back pain.

Physiotherapy

Initially, rest with no riding is usually recommended for horses with ODSP. This duration of rest is dependent on the severity of their back pain and may range from weeks to several months.

After this period of rest, back-strengthening exercises are very important in the treatment of ODSPs in horses. These exercises mainly focus on improving flexibility and improving core strength.¹⁰ Carrot stretches with the head between the front legs and to either side of the front legs have been advocated to improve core strength and flexibility. Pressure can also be placed under the ventral midline to promote dorsiflexion of the spine. Longeing with a Pessoa rig is thought to increase abdominal lift, promote back strengthening, and increase separation of dorsal spinous processes. There have been a few studies that have looked at the effect of abdominal position in relation to the proximity of dorsal spinous processes, and it has been found that the interspinous space between dorsal spinous processes is dynamic and changes with flexion and extension of the spine.¹¹, ¹² As a result, exercises under tack that involve both lateral flexion and movement and ventrodorsal flexion and movement are also key in helping these horses. Proper saddle fit during rehabilitation is of the utmost importance in these horses as thoracolumbar asymmetry from muscle atrophy may predispose them to poor-fitting saddles.²

EXTRACORPOREAL SHOCK WAVE THERAPY (ESWT)

ESWT has been used successfully to decrease back pain associated with ODSPs in horses. One recent study showed that three treatments performed two weeks apart decreased perceived back pain by significantly raising the mechanical nociceptive threshold in horses.¹³ Recommendations have been varied, but more recently a protocol was advocated using 1,000-2,000 pulses total with a combination of both the 35mm and 80mm probe or the recently developed wider focused generator (Xtrode) administered globally or over the clinically sensitive or radiographically abnormal regions.¹⁴ Some use this in conjunction with non-steroidal anti-inflammatories and muscle relaxants such as methocarbamol.

Local Injections

The ODSP can be injected using radiographic and ultrasonographic guidance with corticosteroids (and some use Sarapin) to decrease pain and inflammation at the site. When performing interspinous injections, the author uses radiography to identify a site adjacent to the intended injection sites and then uses ultrasound guidance to place either betamethasone or methylprednisolone extended in volume with sterile saline along the ODSP on either side and into the interspinous ligament. It is extremely difficult to place a needle directly into the interspinous ligament from the dorsal aspect and inject it into the interspinous ligament.

Mesotherapy can also be performed in the epaxial musculature to decrease generalized pain and inflammation.

PEMF (PULSATING OR PULSED ELECTROMAGNETIC FIELDS)

PEMF is widely used amongst owners and trainers for horses that have sore backs along with a variety of other problems. There is very little scientific evidence to validate its use. It is claimed that it increases blood oxygen, activates the lymphatic system, and that acupuncture points are stimulated.¹⁸ This process reduces pain and inflammation and promotes “overall health.” The idea of PEMF is to help relieve inflammation. Pain is thought to be caused by inflammation, so the idea is if we can relieve the pain and swelling, the body can begin to recover naturally.¹⁸

Surgical Intervention For Odsp

Resection of the summits of one or more dorsal spinous processes associated with ODSP was described in 1968.¹⁹ Previously, the general premise of the surgery was to either remove the dorsal spinous process or a portion of it that is associated with ODSP and provide increased space to prevent the DSP from overriding. In more recent years, transection of the interspinous ligament has been performed as an alternative²⁰ to making large incisions and the extensive soft tissue dissection required with removal of the dorsal spinous process.

Resection Of Dsp

Subtotal ostectomy of the overriding dorsal spinous processes have been done under general anesthesia²¹ or in the standing horse.²² , ⁸ The term subtotal ostectomy is a vague description of the procedure because opinions regarding the amount of bone that needs to be removed vary greatly. Some surgeons remove the entire dorsal aspect of the dorsal spinous process that is overriding,²² while others only remove the cranial aspect.²³ The general trend has been to move toward a minimally invasive technique that is thought to maintain most of the soft tissue attachments of the supraspinous ligament and to avoid postoperative morbidity associated with long incisions on dorsal midline. In a more recent retrospective study, 102 horses with ODSP had the cranial margin of the DSP removed through small incisions using bone ronguers.⁸

INTERSPINOUS LIGAMENT DESMOTOMY (ISLD)

Transection of the interspinous ligament between ODSPs has been advocated as an effective treatment for back pain in these horses.²⁰ This is thought to relieve tension on afferent nociceptive receptors located in the ligament insertion, thus abolishing sensation of pain.²⁴ This procedure is usually performed in the standing sedated horse and uses minimally invasive incisions and minimal soft tissue dissection compared to the open subtotal ostectomy techniques.²⁵ The author has modified this technique to include removing a small portion of the cranial ODSP after transecting the interspinous ligament in cases where there is severe bony proliferation.

Evaluation after blocking may be difficult if behavior is intermittent. Some horses may have learned behaviors due to pain aversion that may persist for a time after blocking. In 2006, a paper by Roethlisberger demonstrated that blocking the interspinous ligament in normal horses increased ROM and dorsoventroflexion.

Prognosis

It can be very difficult to interpret the actual prognosis of various treatments because the diagnosis of ODSP varies among the studies as does the definition of success. The presence of concurrent lameness obviously also affects long term prognosis for performance. Most of the studies listed below attempted to exclude horses with concurrent lameness.

• Medical Treatments

■ Retrospective studies evaluating individual conservative or medical treatments are lacking. In a retrospective study comparing medical treatment vs. interspinous desmotoy by Coomer et al. (2012), 89% of 34 horses improved significantly after injection with methylprednisoolone adjacent to the ODSPs but many had recurrent back pain, so the long-term success rate was only 42%.²⁰

• Subtotal or Cranial Ostectomy of the DSP

■ In a retrospective study by Walmsley et al. (2002), 215 horses had partial ostectomy of the DSPs under general anesthesia. The diagnosis was made based on history and clinical signs of back pain, radiographic changes of ODSP and positive response to infiltration of local anesthetic at the ODSP sites. 81% returned to work following surgery and 72% returned to full work.

■ A study by Brink et al. (2014) evaluated 24 horses with ODSP that had standing subtotal ostectomies of ODSPs. Horses were included in the study if they had impaired athletic performance caused by ODSP, radiographic changes associated with ODSP, improved with local anesthetic near the ODSP, and failed medical management. At less than one year, 86% returned to full athletic function and 77% had full athletic function at greater than one year.

■ Jacklin et al. published a retrospective study in 2014 that evaluated a cranial wedge ostectomy of the DSP in 25 horses found to have ODSP. 19 horses were available for long term follow-up of which 15 horses (79%) had complete resolution of clinical signs and were in full work. Three horses were improved and working at a lower level.

■ In a retrospective study, de Souza et al. (2022) evaluated a minimally invasive technique performing a cranial ostectomy in 102 standing horses. Horses were included in the study if they had clinical signs of back pain, radiographic ODSP, and either responded to diagnostic analgesia or had a positive bone scan or improved after injecting the ODSP with corticosteroids.

This study split horses into two groups: those with concurrent lameness issues and those without concurrent lameness issues. Of the 74 horses available for long term follow-up, eight horses had been euthanized (two due to persistent back pain, others due to lameness etc.).

Ȇ 69% of the remaining horses treated with the ostectomy for ODSP that had concurrent lameness returned to the same or higher level of work.

Ȇ 93% of remaining horses treated that did not have concurrent lameness returned to the same or higher level of work.

• Interspinous Ligament Desmotomy (ISLD)

■ In a retrospective paper by Coomer et al. (2012), horses were identified with ODSP with marked signs of back pain by history and physical exam, had no evidence of lameness, and had radiographic ODSP. 95% of 35 horses treated with ISLD had alleviation of clinical signs attributable to back pain and 82% had returned to normal function long term.

■ A study by Prisk et al. (2019),w found that 51 of 56 horses with long term follow-up (median 35 months) returned to some level of work. Of these horses, a lower level of performance was noted in 24 of 51 horses, with 37.5% being from recurrent back pain, 37.5% being from unassociated lameness, and 25% from owners that decided to reduce the horse’s level of riding. The initial diagnosis of ODSP was made based on clinical signs of back pain, radiographic ODSP, and nuclear scintigraphy in 45% of the horses. Radiographic and scintigraphic grades did not impact return to performance.

■ Brown et al. found that nine out of 10 horses that had their ODSPs blocked prior to surgery were improved postoperatively, compared to only four out of eight horses that were not blocked prior to surgery.

■ A retrospective study by Derham et al. compared 159 racing Thoroughbreds with matched cohorts and found that they had improved racing performance. Eight horses developed unilateral neurogenic atrophy of epaxial musculature.²⁶

Pain from ODSP in horses can be difficult to discern from other sources of back pain. Behavior and poor performance that are often linked to ODSP are also commonly associated with a variety of problems including some that are entirely behavioral. It is of the utmost importance to be thorough in your examination to rule out other sources of pain or lameness and not rely entirely on the radiographic presence of ODSP when making a diagnosis. Nuclear scintigraphy and response to diagnostic analgesia are useful in confirming ODSP. There are many effective treatments in managing ODSP in horses. Conservative and medical management is effective in treating many horses, but there is minimal scientific data to help guide us in regard to prognosis. When medical intervention fails, surgical treatments appear to give longer resolution to pain associated with this condition. In general, the overall prognosis for most surgical interventions is around 80%, and there are few complications associated with the surgeries that are used.

References

1. Djernaes JD, Nielsen JV, Berg LC. Effects of x-ray beam angle and geometric distortion on width of equine thoracolumbar interspinous spaces using radiography and computed tomography- a cadaveric study. Vet Radiol Ultrasound. 2017 Mar;58(2):169-175.

2. Greve L, Dyson S. Saddle fit and management: An investigation of the association with equine thoracolumbar asymmetries, horse and rider health. Equine Vet J. 2015 Jul;47(4):415-21.

3. Girodroux M, Dyson S, Murray R. Osteoarthritis of the thoracolumbar synovial intervertebral articulations: clinical and radiographic features in 77 horses with poor performance and back pain. Equine Vet J. 2009 Feb;41(2):130-8.

4. García-López JM. Neck, Back, and Pelvic Pain in Sport Horses. Vet Clin North Am Equine Pract. 2018 Aug;34(2):235-251

5. Mayaki AM, Abdul Razak IS, Adzahan NM, et al. Clinical assessment and grading of back pain in horses. J Vet Sci. 2020 Nov;21(6):e82.

6. Brown KA, Davidson EJ, Ortved K, et al. Long-term outcome and effect of diagnostic analgesia in horses undergoing interspinous ligament desmotomy for overriding dorsal spinous processes. Vet Surg. 2020 Apr;49(3):590-599.

7. Roethlisberger Holm K, J Wennerstrand, U Lagerquist, et al. Effect of local analgesia on movement of the equine back. Equine Vet . 2006 Jan;38(1):65-9.

8. de Souza TC, Crowe OM, Bowles D, et al. Minimally invasive cranial ostectomy for the treatment of impinging dorsal spinous processes in 102 standing horses. Vet Surg. 2022 Jul;51 Suppl 1:O60-O68.

Horses may exhibit a girthy appearance and bite or act resentful during saddling.

9. Zimmerman M, Dyson S, Murray M. Close, impinging and overriding spinous processes in the thoracolumbar spine: the relationship between radiological and scintigraphic findings and clinical signs. Equine Vet J. 2012 Mar;44(2):178-84.

10. Sayers E, Tabor G. An exploration of clinical reasoning and practices used by physiotherapists in the rehabilitation of horses following interspinous ligament desmotomy surgery. Physiother Theory Pract. 2022 Jul;38(7):897-907.

11. O'Sullivan S, McGowan CM, Junnila J, et al. The effect of manually facilitated flexion of the thoracic spine on the interspinous space among horses with impinging dorsal spinous processes of the thoracic vertebrae. Vet J. 2022 Sep 29;289:105909.

12. Pigé C, Masseau I, Bonilla AG. Influence of abdominal elevation on radiographic measurements of the thoracolumbar interspinous spaces in asymptomatic horses. Can Vet J. 2020. Oct;61(10):1101-1105.

13. Trager LR, Funk RA, Clapp KS, et al. Extracorporeal shockwave therapy raises mechanical nociceptive threshold in horses with thoracolumbar pain. Equine Vet J. 2020 Mar;52(2):250-257.

14. Johnson SJ, Richards RB, Frisbie DD, et al. Equine shock wave therapy - where are we now? Equine Vet J. 2022 Oct 9. 1-14

15. Xie H , Colahan P, Ott EA. Evaluation of electroacupuncture treatment of horses with signs of chronic thoracolumbar pain. J Am Vet Med Assoc. 2005 Jul 15;227(2):281-6

16. Haussler K, Holt TN. Spinal Mobilization and Manipulation in Horses. Vet Clin North Am Equine Pract. 2022 Dec;38(3):509-523

17. Sullivan K, Hill A, and Haussler K. The effects of chiropractic, massage, and phenylbutazone on spinal mechanical nociceptive thresholds horses without clinical signs. Equine Vet J. 2008. 40. P14-20

18. https://magnawavepemf.com/faq/

19. Roberts EJ. Resection of thoracic or lumbar spinous processes for relief of pain responsible for lameness and some other locomotor disorders of horses. Proc Am Assoc Equine Practnrs 1968;14:13-30

20. Coomer RPC, McKane SA, Smith N, et al. A controlled study evaluating a novel surgical treatment for kissing spines in standing sedated horses. Vet Surg. 2012 Oct;41(7):890-7

21. Walmsley JP, Pettersson H, Winberg F, McEvoy F. Impingement of the dorsal spinous processes in two hundred and fifteen horses: case selection, surgical technique and results. Equine Vet J. 2002 Jan;34(1):23-8.

22. Brink P. Subtotal ostectomy of impinging dorsal spinous processes in 23 standing horses. Vet Surg. 2014 Jan;43(1):95-8.

23. Jacklin BD, Minshall GJ, Wright IM. 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 Vet J. 2014 May;46(3):339-44.

24.Vandeweerd JM, Desbrosse F, Clegg P, et al. Innervation and nerve injections of the lumbar spine of the horse: a cadaveric study. Equine Vet J. 2007 Jan;39(1):59-63.

25. Prisk AJ, García-López JM. Long-term prognosis for return to athletic function after interspinous ligament desmotomy for treatment of impinging and overriding dorsal spinous processes in horses: 71 cases (2012-2017). Vet Surg. 2019 Oct;48(7):1278-1286

26. Derham AM, O’Leary JM, Connolly SE, et al. Performance comparison of 159 Thoroughbred racehorses and matched cohorts before and after desmotomy of the interspinous ligament. Vet J. 2019 Jul;249:16-2

27. Merrifield-Jones M, Tabor G, Williams J. Inter- and IntraRater Reliability of Soft Tissue Palpation Scoring in the Equine Thoracic Epaxial Region. J Equine Vet Sci. 2019 Dec;83:102812.

Aric Adams, DVM, DACVS

Dr. Aric Adams was a farrier for eight years while obtaining his undergraduate and veterinary degrees. After graduating from the University of MissouriColumbia with a DVM, he completed an internship at an equine hospital in Colorado. Dr. Adams then practiced for one year at an equine hospital in Utah before moving to Florida to begin his surgical training. He completed his surgical residency at the University of Florida in 2005. After working in Tallahassee and Brandon as an associate equine surgeon, Dr. Adams joined the Equine Medical Center of Ocala in 2006. His caseload is primarily made up of lameness, diagnostics, and general surgery, including soft tissue and orthopedic surgeries.