Removal of nucleus pulposus from the intervertebral disc – the use of chymopapain enhances mechanical removal with rongeurs: a laboratory study

Two spines, each of which consists of seven discs, were used for each of the following procedures: (I) unilateral approach using rongeurs alone, (II) bilateral approach using rongeurs alone, (III) unilateral approach using rongeurs followed by chymopapain and (IV) bilateral approach using rongeurs followed by chymopapain.

Sheep lumbar spines were obtained from an abattoir (McIntosh McDonald, Aberdeen). Seven lumbar discs were removed from each spine by sawing through the mid-transverse plane of each vertebra. Pedicles and transverse processes were removed by sawing close to the vertebral body. All extraneous soft tissue (muscle, ligament and fat) was removed to leave an intact intervertebral disc attached to half a vertebra on either side. The resulting segments were wrapped in tissue soaked in physiological saline (9.5 g/L NaCl in de-ionised water) to keep them moist, surrounded by two polythene bags (which were heat-sealed) and stored in a freezer at -40°C. This condition has been shown to have no affect on chemical composition of the disc [15]. Before use, they were thawed overnight at 5°C.

Each specimen was secured using a vice. A 3 mm trocar was made for postero-lateral access to the nucleus (with an entrance angle of 40–60° to the antero-posterior axis), following standard procedure for percutaneous nucleotomy [16, 17]. The trocar was pushed and rotated simultaneously, to perforate the annulus, until a sudden resistance was felt. A 3 mm straight rongeur (Smith & Nephew Richards Inc, Memphis TN, USA) and a 2 mm upbiting rongeur (Smith & Nephew Richards Inc, Memphis TN, USA) were used to remove the nucleus. The procedure was continued until no more nucleus could be removed. The disc was then bisected (in the transverse plane), photographed, and the remaining nucleus removed with rongeurs. Each sample of nucleus removed (before and after bisection) was placed on a weighed piece clingfilm, wrapped and re-weighed; the former was the mass of nucleus removed by the procedure, the latter was the total mass of nucleus.

As much nucleus as possible was removed using rongeurs, and the nucleus removed and weighed as described previously. The specimen was double-wrapped in Clingfilm (to prevent drying) and placed in a 37°C oven for 100 minutes, until it was heated to body temperature. Clingfilm had been shown to prevent detectable water loss in preliminary test. The specimen was then weighed and a needle used to introduce fissures in the remaining nucleus, before injection of chymopapain. The reason for introducing fissures was that chymopapain flows down fissures in prolapsed discs [18]. Chymopapain (30 units; Sigma-Aldrich, Poole, Dorset, UK) was dissolved in de-ionised water (0.1 cm³) at room temperature and used immediately. The dose of chymopapain was based on the work of Kudo et al. [19] which showed it produced no adverse effects on living dogs. This solution was slowly injected into the nuclear cavity using a syringe. The specimen was then returned to the oven for 10 minutes, to keep the chymopapain solution and tissue at 37°C. A bottle brush with flexible nylon bristles (trimmed to an overall diameter of about 10 mm) was used to remove strands of nucleus which were freed by chymopapain (Figure 1). Excess liquid was removed by suction through a plastic tube (2 mm internal diameter) using a pump (model TS400D, Ohaus Corporation, Florham Park NJ, USA). The surface of the specimen was then dried with tissue paper to remove any remaining solution; if excess fluid had not been removed, its mass would have been attributed to remaining nucleus and so given an under-estimate of the mass of nucleus removed. The specimen was then re-weighed. The reduction in mass of the specimen before and after injection of chymopapain was the mass of the nucleus removed by chymopapain. The specimen was then bisected and photographed as before.

The proportion of nucleus removed was the mass of nucleus removed by the procedure divided by the total mass of nucleus. These proportions were compared by ANOVA using Minitab (release 13, Minitab Inc, State College PA, USA) to determine any statistically significant differences between spines and methods for removing the nucleus.