Ovine Lumbar Intervertebral Disc Degeneration Model Utilizing a Lateral Retroperitoneal Drill Bit Injury

Summary

Intervertebral disc degeneration is a significant contributor to back pain and a leading cause of disability worldwide. Numerous animal models of intervertebral disc degeneration exist. We demonstrate an ovine model of intervertebral disc degeneration, utilizing a drill bit, which achieves a consistent disc injury and reproducible level of disc degeneration.

Abstract

Intervertebral disc degeneration is a significant contributor to the development of back pain and the leading cause of disability worldwide. Numerous animal models of intervertebral disc degeneration have been developed. The ideal animal model should closely mimic the human intervertebral disc with regard to morphology, biomechanical properties and the absence of notochordal cells. The sheep lumbar intervertebral disc model fulfils these criteria. We present an ovine model of intervertebral disc degeneration utilizing a drill bit injury through a lateral retroperitoneal approach. The lateral approach significantly reduces the incision and potential morbidity associated with the traditional anterior approach to the ovine spine. Utilization of a drill-bit method of injury affords the ability to produce a consistent and reproducible injury, of precise dimensions, that initiates a consistent degree of intervertebral disc degeneration. The focal nature of the annular and nucleus pulposus defect more closely mimics the clinical condition of focal intervertebral disc herniation. Sheep recover rapidly following this procedure and are typically mobile and eating within the hour. Intervertebral disc degeneration ensues and sheep undergo necropsy and subsequent analysis at periods from eight weeks. We believe that the drill bit injury model of intervertebral disc degeneration offers advantages over more conventional annular injury models.

Introduction

Lower back pain is the leading cause of disability worldwide¹. Lumbar intervertebral disc degeneration associated discogenic pain is considered a significant contributor to lower back pain². There is an increasing demand for reliable animal models of intervertebral disc disease for broadening the understanding of the degenerative process and for the investigation of potential therapies.

Numerous animal models of intervertebral disc degeneration exist³. Animals models used in the investigation of degenerative disc disease range in size from mice⁴, to larger mammals such as dogs⁵, sheep⁶, and non-human primates⁷. Methods used to induce intervertebral disc degeneration can be broadly classified into the categories of mechanical (e.g. intervertebral disc compression⁸ or surgical injury⁶), chemical (e.g. chemical nucleolysis⁵) or, less commonly, spontaneous degeneration (e.g. the sand rat⁹).

Given the complexity of human intervertebral disc degeneration, a perfect animal model does not exist. However, important considerations in choosing an appropriate animal model to mimic this condition closely have been identified³. Such considerations include the absence of notochordal cells (primitive cells with possible progenitor cell function¹⁰ absent from the adult nucleus pulposus in humans, sheep, goats and chondrodystrophic dogs but present in most mammals), similarities in animal and intervertebral disc size relative to humans, comparable biomechanical forces to the clinical condition, mechanistic and ethical considerations³.

Non-human primates meet many of the above criteria. Baboon and macaque models of spontaneous intervertebral disc degeneration have been described^11,12,13. Both species spend large amounts of time in erect or semi-erect postures — a distinct advantage relative to other animal models. However, ethical and practical consideration (e.g. expense, housing, delayed onset of spontaneous degeneration) restrict their use in many institutions.

The ovine spine is an established model of intervertebral disc degeneration, with advantages including cellular, biomechanical and anatomical similarities to the human spine^10,14,15. Despite the quadrupedal stature of sheep the ovine lumbar intervertebral disc is exposed to similar stresses to the human disc¹⁴. The ovine model is also more widely accepted, from an ethical perspective, than non-human primate models. Varied methods have been described to initiate the degenerative process, many of which require direct access to the intervertebral disc. Due to the termination of the spinal cord in the sacral region and ossification of the posterior longitudinal ligament in the ovine lumbar spine, posterior approaches to the intervertebral disc are technically challenging and less commonly used in the sheep¹⁶. The traditional access routes to the sheep lumbar spine, i.e. via anterior or anterolateral approaches, require large abdominal incisions, are fraught with risks of hernia, and damage to internal viscera and neurovascular structures¹⁶. The use of a relatively small lateral incision away from dependent abdominal areas may decrease such risks¹⁷.

We present an ovine model of degenerative lumbar intervertebral disc disease using drill bit injury performed through a minimally invasive lateral approach, and inspired by the work of Zhang et. al¹⁸. The goal of this protocol is to enable a reliable lumbar disc injury model that is readily reproducible, produces a consistent injury, and is safe and well tolerated. This approach is well-suited to investigators seeking to induce a milder degree of lumbar intervertebral disc degeneration than that observed with traditional surgical annulotomy (unpublished data) for the investigation of either intervertebral disc degeneration or regenerative therapies. These findings will be described in a forthcoming publication.

Protocol

The protocol detailed in this manuscript follows the animal care guidelines of Monash University Animal Ethics. Animal ethics approval for this protocol has been granted by Monash University Animal Ethics. Ethics approval number: MMCA/2014/55

1. Sheep Preparation

NOTE: Ewes aged two to four years were used.

1. Fast sheep for 18 h prior to anesthesia. Provide animals with access to water until 6-12 h prior to operation¹⁹.
2. Sedate animals by intravenous injection of medetomidine hydrochloride (0.015-0.020 mg/kg) to facilitate transfer to operating suite.
   NOTE: The medetomidine hydrochloride serves to reduce animal stress and agitation associated with separation from other animals for transfer to the operating suite.
3. Inject thiopentone (10-13 mL/kg) for induction of anesthesia upon arrival to the operating suite.
4. Administer prophylactic intravenous antibiotics (amoxicillin 1 g IV) immediately following thiopentone injection.
5. Intubate sheep using a size 7.5-9 mm (internal diameter) endotracheal tube²⁰.
6. Maintain anesthesia using inhaled isoflurane (2-3%) in 100% oxygen at a flow rate of 2 L/min. Attach a pulse oximeter to the sheep's ear.
7. Closely monitor the sheep's vital signs (heart rate, respiratory rate and oxygen saturation via pulse oximeter and observation) and level of consciousness.
   NOTE: Indicators of light anesthesia such as spontaneous chewing, active regurgitation, and spontaneous movements should prompt increase in the level of anesthesia. Red flag signs indicating urgent lightening of anesthesia include respiratory compromise and severe bradycardia. Rotation of the eye is not a consistent indicator of depth of anesthesia in sheep¹⁹.

2. Disc Level and Incision

1. Collect the surgical tools needed for this procedure: veterinary clippers, 20 mL luer-lock syringe, 21G IV Needle, #4 scalpel handle, #22 scalpel blades, Gillies tissue forceps, Metzenbaum curved dissecting scissors, Deaver retractor, Hohmann retractor blade, 3.5 mm Brad point drill bit, drill bit stop, drill, autoclavable veterinary drill bag, needle holder, 2-0 absorbable synthetic braided sutures, 3-0 absorbable synthetic braided suture and Mayo suture scissors.
2. Prepare the operating suite. Clean the operative table and instrument stand with 70% ethanol. Autoclave all surgical instruments prior to operation. Perform pre-operative anesthetic check.
3. Place the sheep on the operating table in the right lateral position.
4. Using electronic clippers, shave the region defined superiorly by the lower ribs, inferiorly by the iliac bone, medially by the contralateral lumbar transverse processes and approximately 10 cm lateral to the ipsilateral lumbar transverse processes.
5. Palpate the iliac crest, lumbar transverse processes (L1-6) and costo-vertebral angle for the landmarks for surgical incision site. Mark these landmarks with a sterile pen.
6. Prepare the lateral abdomen by disinfecting with chlorhexidine and alcoholic-iodide antiseptic wash.
7. Observe standard surgical aseptic techniques throughout the operation. The surgical team scrubs prior to the operation. Place a sterile fenestrated square drape over the surgical site, and a large sterile square drape on the overhead table.
   1. Sterilize all items to be used within the operative site prior to the operation. Monitor and maintain sterility of the surgical site throughout the operation. Ensure all items introduced into the sterile field are sterile and transferred in a sterile fashion.
8. Use surgical loupe magnification and a headlight to facilitate visualization during the surgical procedure.
9. Make a longitudinal incision using the #22 scalpel blade attached to the #4 scalpel handle parallel to and 1 cm anterior to one to two lumbar transverse processes above and below the intervertebral disc levels of interest.
   NOTE: Further information regarding incision planning can be found in the discussion.
10. Use the monopolar diathermy to divide the underlying subcutaneous tissue and the lateral aspect of the abdominal wall musculature; direct the dissection towards the tips of the lumbar transverse processes above and below the intervertebral discs of interest.
11. Divide the thoracolumbar fascia longitudinally at its attachment to the transverse processes.
12. Visualize and preserve the underlying quadratus lumborum, psoas muscles and the traversing neurovascular bundles.
13. Maintain hemostasis through the procedure using diathermy.
14. Sweep the fingers between the plane of the peritoneum and posterior abdominal wall musculature at the exposed intervertebral disc levels to perform digital blunt dissection.
15. Retract the quadratus lumborum and psoas muscles posterolaterally using a Deaver retractor to further expose of the intervertebral discs.
16. Palpate for the concave intervertebral bodies and the convex intervertebral discs.
17. Position the retractors immediately over the discs and take care to ensure lumbar vessels are not damaged.
18. Using surgical loupe magnification with a headlight, identify the lumbar vessels which are located approximately 1 cm caudal to the inferior endplate.
19. Perform an intraoperative lateral X-ray to confirm the disc level.²¹
    Note: Radiograph settings: 47kV; 4 mAs²¹
20. Depending on the disc levels desired, expose the intervertebral disc by separating the surrounding structures and attachments as below.
    1. For levels L3/4 and above, sweep aside the muscular attachments over the disc using digital blunt dissections.
    2. For levels L4/5 and below, sharply divide the thicker tendinous muscular attachments over the disc using bipolar diathermy and scissors.
       NOTE: L6/S1 disc can be difficult to access due to obstruction by the iliac crest. If access cannot be accomplished via the lateral approach an anterior approach may be utilized.

3. Drill Bit Injury

NOTE: Pre-operative planning includes the allocation of injury/treatment levels and control levels. Further information regarding level allocation can be found in the discussion.

1. Define the drill bit entry point by observing the left lateral and anterior extremities of the intervertebral disc.
   NOTE: The entry point is located at the midpoint of this left anterolateral quadrant (defined by the anterior and lateral extremities of the disc). The drill bit is inserted at this entry point with a trajectory aimed towards the center of the intervertebral discs and directed slightly cranial to perpendicular.
2. Fit a Brad-point drill bit into the power drill. Ensure the diameter of the drill bit is slightly less than the intervertebral disc height i.e. ~3.5 mm for lumbar intervertebral disc in 60-70 kg sheep.
   1. Apply a drill bit stop to provide an unprotected drill bit length of approximately half the diameter of the lumbar intervertebral disc i.e. ~12 mm for lumbar intervertebral discs in 60-70 kg sheep.
3. Apply the drill bit to the entry point and direct it in a trajectory slightly cranial to the center of the intervertebral disc. The slight cranial angulation is to minimize the risk of endplate injury.
4. Advance the drill bit slowly into the intervertebral disc with the drill on low power for 1 s. Adjust the trajectory in a slight cranial or caudal fashion if excessive resistance is encountered
   NOTE: Such excessive resistance likely indicates contact with the endplate.

4. Closure

1. Once hemostasis is achieved, irrigate the wound with Ringers' solution.
2. Perform layered closure, preferably using 2-0 absorbable synthetic braided sutures to the lateral abdominal wall tissues and continuous 3-0 undyed absorbable synthetic braided subcuticular suture to the skin.

5. Post-operative Management

1. Place a fentanyl transdermal patch (75 µg/h) in the inguinal region for post-surgical analgesia for 3 days.
2. Additionally, use intravenous buprenorphine (0.005-0.01 mg/kg) for top-up analgesia if needed.
3. Cease the inhalational anesthetic. When spontaneous breathing occurs, remove the endotracheal tube.
4. Allow the animal to recover in a holding cage under constant observation.
   NOTE: The animal should not be left unattended until it has regained sufficient consciousness to retain sternal recumbency.
5. Once the animal is fully alert and standing, re-introduce food and water. Once fully recovered, return the animal to its operative facility holding pen with other animals.
6. Monitor closely for 24 h and continue observation for one week. Monitor for evidence of post-surgical pain or distress.
   NOTE: Post-operative transdermal fentanyl patch applied for three days should provide sufficient analgesia. Additional analgesic requirements should prompt animal review.
7. Feed the sheep normally, and allow the sheep to carry out normal activities without restriction. Observe the sheep for any evidence of neurological deficit such as lameness.
   NOTE: The intervertebral disc defect produced by the drill bit injury method is on the anterolateral aspect of the disc and the injury depth is limited by the drill bit stop to the mid nucleus. As the neural elements are located posterior/posterolateral to the intervertebral disc, the risk of neural compromise secondary to symptomatic nucleus pulposus is remote. This anatomical characteristic of the model precludes the use of neurological examination to distinguish intervertebral disc degeneration with and without nucleus pulposus herniation.
8. Return the sheep to the university farm to await euthanasia and necropsy at the end of the experimental period.

6. Euthanasia

1. Perform sheep euthanasia at an appropriate time interval following drill bit intervertebral disc injury.
2. Inject intravenous pentobarbitone sodium (>100 mg/kg) for euthanasia.

Results

Pre-operatively, sheep underwent baseline 3T magnetic resonance imaging (MRI) for assessment of underlying intervertebral disc morphology and degeneration. Sheep underwent additional intra-operative lateral radiography for confirmation of intervertebral disc level and calculation of disc height index. A pre-operative sagittal plane slice from 3T MRI and an intra-operative radiograph are demonstrated in Figure 1.

Discussion

This minimally invasive lateral access approach is efficacious and safe with no post-operative herniae, abdominal wound dehiscence or infection observed in this series. Use of the drill bit intervertebral disc injury model with a depth stop provides a reproducible method of inducing a consistent intervertebral disc injury of known dimension (i.e. a 3.5 mm diameter x 12 mm depth injury in this study). In our experience, this method produces a less severe degree of disc degeneration than that observed in conventio...

Materials

Name	Company	Catalog Number	Comments
Medetomidine Hydrochloride (10 mL Injection)	Therapon/Zoetis	PFIDOM10	Multiple suppliers: Zoetis/Ilium
Thiopentone	Troy	Triothiopentone	Multiple suppliers: Neon Laboratories, Jagsonphal Pharmaceuticals
Isoflurane (2 - 3 % in oxygen)	Baxter	AHN3636	Multiple suppliers: Baxter/VetOne
Amoxicillin parenteral	GlaxoSmithKline	JO1CA04	Multiple suppliers: GlaxoSmithKline/Merck
Bupivacaine (0.5% Injection 20 mL)	Pfizer	005BUP001	Multiple suppliers: Pfizer/AstraZeneca
PVD Iodine Solution	Jurox	61330	Multiple suppliers: Jurox/Orion
Chlorhexidine 5%w/v	Jurox	Chlorhex C 5L (SCRUB)	Multiple suppliers: Jurox/Pfizer
Transdermal Fental Patch (75 μg/h)	Janssen-Cilag	S8-Dur7.5	Multiple suppliers: Sandoz
Buprenorphine iv	Jurox	504410	Multiple suppliers: LGM Pharma
Atipamezole (Antisedan 0.06 mg/kg - 0.08 mg/kg)	Zoetis	PFIANT10	Multiple suppliers: Ilium
Oster Golden A5 2-Speed Clippers	Oster	078005-140-003	Oster
20 mL luer lock syringe	Terumo	6SS+20L	Multiple suppliers: Medshop Australia/Terumo
21 G IV needle	Terumo	SG3-1225	Multiple suppliers:Medshop Australia/Terumo
#4 scalpel handle	Austvet	AD010/04	Multiple suppliers: Austvet/SurgicalInstruments
#22 scalpel baldes	Austvet
Gillies tissue forceps	Austvet	AB430/15	Multiple suppliers: Austvet/SurgicalInstruments
Metzenbaum curved dissecting scissors	Austvet	AC101/14	Multiple suppliers: Austvet/SurgicalInstruments
Deaver retractor	Surgical Instruments	23.75.03	Multiple suppliers: Surgical Instrument/Austvet
Hohmann retractor	Austvet	KA173/35	Multiple suppliers: Austvet/SurgicalInstruments
Mayo suture scissors	Austvet	AC911/14	Multiple suppliers: Austvet/SurgicalInstruments
Needleholder 14 cm	EliteMedical	18-1030	Multiple suppliers: EliteMedical/Austvet
CMT 3.5 mm Brad-Point Drill	Carbatec	516-035-51	Multiple suppliers: Southeast Tool/Carbatec
Drill Bit Stop 4 mm	Drill Warehouse	20121600	Multiple suppliers: Amazon
Bosch 10.8 V Cordless Angle Drill	Get Tools Direct	GWB10.8V-LIBB	Multiple suppliers:Bunnings/Get Tools Direct
Autoclavable veterinary drill bag	AustVet	DRA043-AV	AustVet
2-0 absorbable synthetic braided sutures	Ethicon	VCP335H	Ethicon
3-0 absorbable synthetic braided sutures	Ethicon	VCP232H	Ethicon
Siemens 3 Tesla Skyra Widebore MRI	Siemens	N/A	Siemens
9.4 Tesla Agilent (Varian) MRI	Agilent Technologies	N/A	Agilent Technologies
Atomscope HF 200 A Radiogaph	Radlink	330003A	Multiple Suppliers: Radlink/DLC Australia
Veterinary Pulse Oximiter	DLC	192500A	Multiple suppliers: DLC Australi Pty Ltd/AustVet