Development of a minimally invasive approach to equine cervical articular facet joints for placement of an ND:YAG LASER

Abstract

The purpose of this study was to develop a minimally invasive approach to equine cervical articular facet joints for application of an ND:YAG LASER and to evaluate the effects of the laser fiber on the surrounding tissue. Under general anesthesia, an ND:YAG LASER was used to apply 2000 J of energy to 5 cervical articular facet joints in 3 horses (15 joints total). Horses were euthanized and the cervical facets and spinal cords were examined grossly and histologically. Gross pathology of the articular facets revealed evidence of articular cartilage charring and diffuse roughening of the surface. Histopathology confirmed coagulative necrosis. This novel technique allowed access to the cervical articular facet joints with the ND:YAG LASER and has the potential to allow performance of a minimally invasive facilitated ankylosis procedure. Further validation in sedated, standing horses is required to establish safety and efficacy of this technique.

Introduction

Osteoarthritis of the cervical articular facet joints is a performance limiting condition in horses of various sport disciplines. Degenerative changes between the 6th and 7th cervical vertebrae have been reported on postmortem examination in 50% of normal mature horses (1). Osteoarthritis of the cervical articular facet joints causes enlargement of the joint capsule and periarticular new bone formation (2). These degenerative changes can lead to neck pain, lameness, or subsequent spinal cord compression and ataxia (1,3,4). Cervical stenotic myelopathy secondary to osteoarthritis of the caudal cervical articular facets is more common in older horses; younger horses (< 5 years) are more likely to have a developmental cervical malformation condition (5,6).

Conservative management of caudal cervical articular facet osteoarthritis may include local intra-articular injection of corticosteroids and administration of a systemic non-steroidal anti-inflammatories (7,8). A 25% to 71% subjective improvement in performance or return to normal function was reported based on owner assessment following intra-articular cervical facet injections for treatment of osteoarthritis (8). Aside from this report, no objective or quantitative data are available to determine the long-term success rate of this conservative therapy. The surgical treatment of choice for cervical stenotic myelopathy is ventral interbody fusion. This procedure requires specialized equipment, and an experienced surgeon and anesthesia team to ensure a smooth recovery. Complications reported following ventral interbody fusion include seroma formation, infection, fracture, migration of the implant, laryngeal hemiplegia, and Horner’s syndrome (9).

Horses with interbody fusions between the 6th and 7th cervical vertebrae have a worse prognosis for return to use and higher incidence of complications during recovery from anesthesia compared with horses treated at C3 to C4 (10,11). Surgical success rates reported by most surgeons are approximately 60% (9–12) and success is commonly defined as a 1 to 2 grade improvement in ataxia.

LASER-facilitated ankylosis of the distal tarsal joints and pastern joints in horses has been reported with favorable outcome (13–15). Previous studies have found that LASER treatment heats chondrocytes above 50°C, resulting in cell death (13,14). This method was reported to facilitate ankylosis of the distal tarsal joints and return 23/24 Western performance horses to soundness following surgery (13). The successful use of a LASER in combination with 3 parallel 5.5-mm screws placed in lag fashion for arthrodesis of the proximal interphalangeal joint has also been described. This technique is particularly useful in horses with significant osteoarthritis within the joint and allows for immediate improvement in comfort of the treated horses (16). The use of diode or ND:YAG LASERs within joints has been postulated to damage the nervous tissue within the joint preventing the transmission of a painful stimulus and providing an immediate decrease in pain (16).

The use of LASER on cervical articular facet joints has not been reported. LASER-facilitated ankylosis could aid in immobilization of the joint which may prevent dynamic compression of the spinal cord and decrease local inflammation. A minimally invasive technique could potentially be done in a standing sedated animal, reducing the complications associated with the previously mentioned interbody fusion techniques. However, the first step is to develop a minimally invasive approach to the cervical articular facet joints for placement of an ND:YAG LASER and evaluate for potential adverse effects on the spinal cord.

The purpose of this study was to i) develop a minimally invasive approach to the cervical articular facet joints; and ii) determine the gross cartilage destruction that can be obtained using this technique. We hypothesized that a minimally invasive procedure could be developed to place an ND:YAG LASER fiber safely and consistently within the articular facet joints and that the ND:YAG LASER within the joint would cause pathology to the articular surface of the joint and not to the spinal cord.

Materials and methods

This study was performed in 15 cervical facet joints from 3 horses (ages 7 to 19 y, weighing between 400 and 550 kg) donated for reasons unrelated to cervical disease. The horses were randomly assigned to have the procedure carried out on the cervical articular facet joints of either the right or left side of the neck. The horses had not received any non-steroidal anti-inflammatory drugs in the last 7 d and did not receive any anti-inflammatory drugs before the treatment. This protocol was reviewed and approved by the Michigan State University Institutional Animal Care and Use Committee.

Anesthesia

After sedation with xylazine hydrochloride (Akorn Animal Health, Lake Forest, Illinois, USA), 1.1 mg/kg body weight (BW), IV, general anesthesia was induced with ketamine hydrochloride (Akorn Animal Health), 2.2 mg/kg BW, IV, and diazepam (Hospira, Lake Forest, Illinois, USA), 0.1 mg/kg BW, IV. The horses were randomly placed in either left or right lateral recumbency and maintained on isoflurane in oxygen. Horses were monitored during anesthesia with conventional techniques by an experienced anesthesia technician. Following completion of the procedure, the horses were maintained under general anesthesia for 60 min to allow time for an acute inflammatory response to the LASER application and were then euthanized using pentobarbital (Vortech Pharmaceuticals, Dearborn, Michigan, USA), 0.22 mg/kg BW, IV.

LASER technique

The lateral aspect of the neck from C2–3 to C6–7 was clipped and aseptically prepared. The facet joints were located by ultrasound as described by Berg et al (17) and Nielson et al (18) using a General Electric LOGIQ P5 veterinary ultrasound system (GE Healthcare, Chicago, Illinois, USA) with a 10-4 MHz microconvex curvilinear transducer. The transducer was positioned transverse to the line of the vertebrae and the widest joint space and best access to the joint was located prior to injection and imaged using 10 MHz. An 18-gauge, 8.89-cm spinal needle was inserted into the C2–3 facet joint under ultrasound guidance. The joint was then distended with 5 to 15 mL of sterile saline (until back pressure was detected), the spinal needle was removed, and a 14-gauge, 12.7-cm intravenous catheter (Abbott Animal Health, Stephenville, Texas, USA) was then ultrasonographically guided into the distended facet joint. The stylet was removed and the 1.05 mm × 2.5 m ND:YAG LASER contact fiber (Surgical Laser Technologies, Montgomeryville, Pennsylvania, USA) was inserted through the 14-G catheter and advanced 5 mm beyond the sleeve, after which 2000 J were then applied to the cervical facet joint. This procedure was repeated unilaterally for each cervical facet joint beginning at C2–3 and extending to C6–7.

Gross and histologic pathology evaluation

Each horse was presented for necropsy within 2 h of euthanasia. Cervical vertebrae were disarticulated, and the spinal cord segment within the vertebral canal of each vertebra was removed; a labeled tag was clipped to the left cranial aspect of the dura mater of each cord segment to retain anatomic perspective. Spinal cord segments were placed in 10% neutral buffered formalin for at least 7 d so that the tissues would be adequately fixed before processing for histopathologic examination.

Two transverse and 2 longitudinal sections of spinal cord were cut from both ends of each segment at the level of each intervertebral disc from C2–C3 through C6–C7. The 4 segments spanned a length of 2.5 cm centered over the disc. Thus, a total of 20 cervical spinal cord sections were prepared from each of the 3 necropsied horses.

The disarticulated cervical vertebrae were examined for lesions, and all cranial and caudal articular facets were photographed. A subjective grading scale was developed to determine articular cartilage surface damage; Grade 1 — the cartilage surface has diffuse cobblestone surface texture with no char marks; Grade 2 — the cartilage has cobblestone appearance with a char mark on either the cranial or caudal facet; Grade 3 — cartilage has cobblestone appearance with char mark on both the cranial and caudal facet (Figure 1).

Figure 1.

Gross pathology of the facet joints demonstrating the various grades following laser application. In all images, the left facet joint is the non-treated side and the right is the laser treated side. A — Grade 1 with the cranial and caudal articulations. B — Grade 2 with the cranial and caudal articulation. C — Grade 3 with the cranial and caudal articulation.

Articular facets were then removed from the vertebrae at their bases with a Hobart band saw (model 5K49SH22208; Hobart, Troy, Ohio, USA) and placed in formalin fixative for at least 2 wk. Following fixation, thin (5- to 6-mm wide) sections of the facets were cut using a Techcut5 Precision High Speed saw (Allied High Tech Products, Rancho Dominguez, California, USA) through standard sites of the articular surfaces and at the margins of articular surfaces where gross lesions were noted.

The thin bone sections were placed in RapidCal decalcification solution (BBC Biochemical, Mount Vernon, Washington, USA) for 24 h to soften them in preparation for histopathology. Transverse and parasagittal longitudinal sections were cut from the ends of the collected spinal cord segments, from the caudal aspect of the C2 segment through the cranial aspect of the C7 segment. Decalcified bone and spinal cord sections were then processed routinely, embedded in paraffin, sectioned at 5 μm, and stained with hematoxylin and eosin.

Results

Ultrasound-guided LASER technique

Placement of the spinal needle and distension of the cervical articular facet joints were easily performed. The ultrasound image of each cervical facet joint was easily obtained and the 18-gauge spinal needle was directed into the joint space and distended with saline (Figure 2). With distension of the joint, the catheter was visualized entering each articular cervical facet joint and the LASER fiber passed readily through the lumen of the 14-gauge catheter into the joint space. In most cases, a smoke plume and bubbling joint fluid could be seen exiting the external portion of the catheter (Figure 3). Horse 1 had a mild amount of periarticular irregularity of the C5–6 and C6–7 articular facets noted on ultrasound examination; however, these joints were still easily accessible with the catheter and LASER fiber. The total procedure time per joint was approximately 5 min.

Figure 2.

Ultrasound image of cervical facet joint C3–C4. White arrow indicates catheter entry into the joint capsule (black arrow).

Figure 3.

Smoke plume escaping from 14-gauge catheter with fiber wire in place in the articular facet joint.

Gross pathology

Gross pathology performed on Horse 1 revealed dark charred marks on the articular cartilage of all left cervical articular facet joints except C2–C3. The charred marks between C5–C6 and C6–C7 involved both the cranial and caudal portions of the joint (Grade 3) while only the cranial most facet of C4–C5 was affected and only the caudal most facet of C3–C4 was affected (Grade 2).

Gross pathology performed on Horse 2 revealed charred marks on the articular cartilage of all left cervical articular facet joints. The charred marks between C4–C5, C5–C6, and C6–C7 were located on either the cranial or caudal facets (Grade 2), while C3–C4 and C2–C3 had no gross pathology of the cartilage.

Gross pathology performed on Horse 3 revealed evidence of Grade 1 articular damage including diffuse granular roughening or cobblestone appearance of the cranial and caudal facets of the right cervical articular facet joints C3–C4, C4–C5, C6–C7. There was 1 Grade 2 lesion between C2–C3 and 1 Grade 3 lesion between C5–C6.

Overall, using the grading scale, there were 3 Grade 3 lesions, 6 Grade 2 lesions, 3 Grade 1 lesions, and 3 joints that had no evidence of pathology from the LASER. In addition to these findings, gross evidence of epidural hemorrhage was present in the caudal 2- to 3-cm segment of spinal cord within the C3 vertebrae, C4 and C5 vertebrae, primarily on the right side.

Spinal cord histopathology

Horse 1 (treated on the left side) had no significant histologic lesions in the spinal cord. Horse 2 (treated on the left side) had a few extravascular neutrophils loosely scattered in 2 areas of the leptomeninges of the cranial C5 spinal cord segment, and in 1 area of the leptomeninges of the caudal C6 spinal cord. There were no other significant histologic lesions in the spinal cord. Horse 3 (treated on the right side) had a few extravascular neutrophils that were loosely scattered in 1 area of the leptomeninges of the caudal C5 spinal cord segment, and in 2 areas of the leptomeninges of the caudal C6 spinal cord segment. There were no other significant histologic lesions in the spinal cord.

Bone histopathology

Due to similar gross findings on all articular facets of all horses, histopathology of the articular facets was performed on Horse 1 only. Two bone sections, cut cranially to caudally, were prepared from the middle of the articular surfaces of facets on the treated left side; 1 section was 7 to 10 mm more medial than the other section. One similarly cut (made cranially to caudally) bone section was prepared from the middle of the articular surface of each corresponding facet on the untreated right side. Additional bone sections were cut from areas where the LASER treatment had created a grossly apparent, brown-black mark or depression in the articular facet; these lesions generally were at the lateral margins of the articular surfaces, or just medial to the lateral margin.

Acute or recent bone lesions were observed only where grossly apparent, brown-black marks were noted on articular cartilaginous surfaces. These lesions were located on the: 1) left caudal facet of C3; 2) left caudal facet of C5; 3) left cranial facet of C6; 4) left caudal facet of C6; and 5) left cranial facet of C7. At all of these sites, the articular cartilage was thinner than the surrounding cartilage, had an irregular surface contour, and was granular and amphophilic, indicating coagulative necrosis. At most of these sites, the articular cartilaginous surface was coated with a thin layer of orange-brown to black material (LASER treatment residue). On the left caudal facet of C3 and the left cranial facet of C6, the coagulative necrosis extended into the subchondral bone. There was no evidence of inflammation at any of these sites. Mild irregularities in the articular cartilage were noted on some articular facets, with thinning or extension of a thin band of fibrous tissue across the surface being most common. These changes were deemed to be chronic and degenerative, and gross images of the articular facets supported this interpretation.

Additional soft tissue histopathology

Skeletal muscle and other soft tissues near the articular facets of the C2–C3 and C3–C4 intervertebral joints in Horse 3 showed small dark brown to black marks on the treated right side, and these tissues were collected for histopathology. Some skeletal myofibers were hypereosinophilic (indicating mild degeneration, with cross-striations still evident), and were adjacent to small, irregular aggregates of orange-brown to black material (associated with the LASER treatment). Small numbers of neutrophils were marginated in a few nearby capillaries, or were extravasated into the surrounding connective tissue. One section from the C3–C4 area included adipose tissue and what was believed to be synovium, and these tissues contained small numbers of extravasated, loosely scattered neutrophils as well.

Discussion

This is the first study to describe access to the cervical articular facets joints with an ND:YAG LASER. The effect of the LASER on the articular surface, spinal cord, and surrounding soft tissue was described postmortem by gross pathology as well as histopathology. Our study shows that an ND:YAG LASER can be placed into the equine cervical articular facet joints. Whether the damage caused by the LASER would be significant enough to induce ankylosis of the cervical facets would need to be examined in live horses recovered from general anesthesia or in horses that had the procedure performed under standing sedation. This approach requires further validation and outcome evaluation in live horses before it can be recommended in clinical practice.

One of the main advantages of LASER facilitated ankylosis is its potential to be a minimally invasive method of cervical articular facet fusion in older horses with osteoarthritis of the caudal cervical facet joints. Clinically, this may provide a useful treatment option for horses with chronic osteoarthritis that is non-responsive to conservative medical management (intra-articular corticosteroid injections). Additionally, while this proof of concept study was carried out under general anesthesia, ultrasound-guided insertion of the LASER fiber into the cervical facet joints in the standing horse would offer the advantage of avoiding general anesthesia in an older horse that may have neurologic deficits.

In our study there was gross evidence of charring on the articular cartilage that corresponded to evidence of coagulative necrosis histopathologically in the affected sections. There was no evidence of inflammation present and this was likely a reflection of the limited time scale chosen for this project; horses were alive for only 60 min after LASER treatment. The subjective grading scale was used to try and quantify damage to the articular surface. Histopathology and cartilage staining would be required to determine the true effect on the cartilage.

Ankylosis via LASER has been reported with good outcome in the distal tarsal joints (13) and it has been suggested that LASER energy causes vaporization of synovial fluid followed by chondrocyte death, thereby allowing ankylosis, and that contraction of the joint capsule due to heating of the collagen may provide additional joint stability (13–15). Pathologic findings on LASER treated distal tarsal joints by Scruton et al (15) were similar to those in the current study. In the Scruton study, there was evidence of necrotic char and subchondral bone blanching grossly and histopathologic evidence of fibrous tissue at the laser-treated areas that contained small amounts of woven bone bridging across the joint surfaces at 5 mo after LASER treatment. That same study measured chondrocyte death with confocal LASER microscopy in conjunction with vital cell staining and found significantly more chondrocyte death following LASER treatment compared with intra-articular drilling (15).

In addition to its efficacy, LASER-facilitated ankylosis in the distal tarsal joints has been reported to be minimally painful during the post-operative period compared with surgical drilling and intra-articular sodium monoiodoacetate injection (13–15,19). This may be due to an effect of the LASER on nerves within the subchondral bone, synovium, and joint capsule (13,14,19) and would be advantageous in horses with painful osteoarthritis of the caudal cervical articular facets.

There are several potential concerns that should be considered if the technique is to be used in clinical cases. There was a significant amount of hemorrhage within the epidural space in Horse 3. Hemorrhage, and inflammation in and surrounding the spinal cord could have grave consequences. The use of ultrasound to guide entry into the articular facet joints by an experienced operator is critical to ensure the LASER is placed correctly. In addition, the long-term objective of facilitating ankylosis relies on the body’s response to the LASER by articular fusion. This fusion requires monitoring live horses over a period of months to years. Assuming fusion occurs, there may also be a risk of exuberant periosteal reaction and periarticular bone formation. If excess bone formation protrudes into the spinal canal, there is a potential for compression of the spinal cord and increased neurologic deficits.

There are several limitations of this study. The horses selected for this study were only required to be free of cervical neck pain; no radiography or ultrasonography was performed to evaluate the cervical vertebrae for pathology. Additionally, these horses were essentially free of osteoarthritis and entry to the cervical articular facet joints was relatively easy; this may be more challenging in clinical cases with significant joint pathology. The placement of the LASER fiber within the joint was not difficult; however, we cannot be sure of the exact location of the fiber within the joint and we can only assume the energy of the LASER is diffusely affecting the entire joint. An ND:YAG LASER was chosen based on availability of the LASER at our institution; however, a diode LASER could theoretically be used with similar results.

In conclusion, our results show that an approach to the cervical articular facet joints and introduction of a LASER fiber is possible in anesthetized horses. The effect on the cartilage is unknown and would require further study. This effect could be anticipated to facilitate ankylosis over a period of months to a year when performed in live horses. Further study is required in live horses to determine the safety and efficacy of this technique. CVJ