Get Off My Back! Managing the Horse With Overriding Dorsal Spinous Process
Written by 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.
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 angles.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 the range of motion and dorsoventral flexion and extension⁷ which may complicate interpretation. Others also have used positive responses to injections with corticosteroids adjacent to the ODSP to help verify them as the source of back pain.⁸ The 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.
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 the 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.
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 using1,000-2,000 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.
The ODSP can be injected using radiographic and ultrasonographic guidance with corticosteroids (and some useSarapin) 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.
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 the removal of the dorsal spinous process.
Rejection of DSP
Subtotal osteotomy of the overriding dorsal spinous processes has been done under general anesthesia²¹ or in the standing horse.²²,⁸ The term subtotal osteotomy 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 the dorsal midline. In a more recent retrospective study, 102 horses with DSP had the cranial margin of the DSP removed through small incisions using bone rongeur.⁸
Interspinous Ligament Desmotomy (ISLD)
Transection of the interspinous ligament between ODSP 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 the 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 osteotomy 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.
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 the long-term prognosis for performance. Most of the studies listed below attempted to exclude horses with concurrent lameness.
- 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 methylprednisolone adjacent to the ODSPsbut 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 osteotomy 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 the 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 osteotomies of ODSPs. Horses were included in the study if they had impaired athletic performance caused by ODSP, radiographic changes associated with ODSP, improved with a local anesthetic near the ODSP, and failed medical management. At less than a 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 osteotomy 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 for performing a cranial osteotomy in 102 standing horses. Horses were included in the study if they had clinical signs of back pain, radiographicODSP, 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 the74 horses available for long-term follow-up, eight horses had been euthanized (two due to persistent back pain, others due to lameness, etc.).Ȇ69% of the remaining horses treated with hepatectomy for ODSP that had concurrent lameness returned to the same or higher level of work.Ȇ93% 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, radiographicODSP, 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 ODSPin 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.
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About the Author
Dr. Aric Adams was a farrier for eight years while obtaining his undergraduate and veterinary degrees. After graduating from the University of Missouri-Columbia 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.Tags: Practitioner, veterinary medicine