We have studied, prospectively, 116 patients with

Recovery of motor deficits after microdiscectomy for lumbar disc herniation F. Postacchini, G. Giannicola, G. Cinotti From the University La Sapienza, Rome, Italy We have studied, prospectively, 116 patients with motor deficits associated with herniation of a lumbar disc who underwent microdiscectomy. They were studied during the first six months and at a mean of 6.4 years after surgery. Before operation, muscle weakness was mild (grade 4) in 67% of patients, severe (grade 3) in 21% and very severe (grade 2 or 1) in 12%. The muscle which most frequently had severe or very severe weakness was extensor hallucis longus, followed in order by triceps surae, extensor digitorum communis, tibialis anterior, and others. At the latest follow-up examination, 76% of patients had complete recovery of strength. Persistent weakness was found in 16% of patients who had had a mild preoperative deficit and in 39% of those with severe or very severe weakness. Muscle strength was graded 4 in all patients with persistent weakness, except for four with a very severe preoperative deficit affecting the L5 or S1 nerve root. They showed no significant recovery. Excluding this last group, the degree of recovery of motor function was inversely related to the preoperative severity and duration of muscle weakness. The patients subjective functional capacity was not directly related to the degree of recovery except in those with persistent severe or very severe deficit. J Bone Joint Surg [Br] 2002;84-B:1040-5. Received 5 November 2001; Accepted after revision 3 April 2002 F. Postacchini, MD, Professor and Chairman G. Giannicola, MD, Orthopaedic Surgeon G. Cinotti, MD, Assistant Professor Department of Orthopaedic Surgery, University La Sapienza, Piazzale le Aldo Moro 5, 00187 Rome, Italy. Correspondence should be sent to Professor F. Postacchini. 2002 British Editorial Society of Bone and Joint Surgery 0301-620X/02/712948 $2.00 Few studies have described the incidence of muscle weakness and its recovery after conventional surgery 1-6 or microsurgery 7 for herniation of a lumbar disc. Even fewer investigations, carried out on patients who had conventional surgery, 8-12 have considered the postoperative restoration of muscle power and all but one 12 have focused only on the muscles supplied by the L5 nerve root 8,9 or extensor hallucis longus (EHL). 10,11 There have been contradictory results concerning the proportion of patients who recover and the degree of recovery. 13 Little and conflicting information is available on the factors which may affect the postoperative restoration of muscle strength. Furthermore, no investigation has analysed the patient s functional disability resulting from a persistent muscle weakness. We have therefore attempted to evaluate the long-term results of microsurgical discectomy in patients with a herniated disc at any level in the lumbar spine in order to determine the incidence of preoperative muscle weakness and the rate and degree of recovery after surgery, to investigate possible factors which may affect the recovery, and to analyse whether a persistent muscle deficit is associated with any subjective functional disability. Patients and Methods Between 1991 and 1997, microsurgical discectomy was undertaken by the senior author (FP) on 508 patients who had herniated lumbar discs without significant spinal stenosis. Preoperative clinical examination, consistently carried out by the same orthopaedic surgeon, revealed 138 patients (27%) with weakness of one or more muscles in the lower limb. Loss of muscle strength was evaluated according to the classification of the Medical Research Council (MRC), 14 which grades muscle strength on a scale of 0 (paralysis) to 5 (normal strength). The strength was assessed manually for all muscles except triceps surae which was tested both manually and by asking the patient to rise on tiptoe on each leg separately. No patient had complete paralysis of any muscle or the cauda equina syndrome. In no patient did muscle weakness become worse after surgery. The preoperative diagnosis was made by CT or MRI or both. Electromyography (EMG) was performed in a small number of patients usually to obtain an objective demonstration of any neural deficit for medicolegal reasons. At the time of surgery, the type of herniation (contained, extruded or sequestrated) and the site (posterior or lateral, i.e. intraforaminal or extraforaminal) were recorded. All patients 1040 THE JOURNAL OF BONE AND JOINT SURGERY

RECOVERY OF MOTOR DEFICITS AFTER MICRODISCECTOMY FOR LUMBAR DISC HERNIATION 1041 were examined after an interval of one, two, three, four and six months after surgery. Each follow-up examination was carried out by the senior author and, independently, by the surgeon who had examined the patient preoperatively. Muscle strength. To assess the strength of the EHL, the examiner (right-handed), stood on the patient's right side, resisting foot dorsiflexion with the dorsum of his right hand, and tested extension of the big toe with his left middle finger, or both the index and middle finger, placed at the level of the distal phalanx of the big toe. 15 This manoeuvre allows the EHL to exert its maximal strength in isolation. The strength of tibialis anterior was tested with the patient supine and the knee flexed at 30. In this position the strength of dorsiflexion is less than that with the knee extended and muscle weakness is more easily detected and graded. 15 The assessment in this position was confirmed by measuring the strength of dorsiflexion with the patient sitting on the edge of the bed. To test the strength of the peronei, the patient was asked to dorsiflex the foot and then to evert it against the resistance of the examiner s hand, who, at the same time, grasped the patient s ankle with the opposite hand to prevent external rotation of the whole limb. 15 Extensor digitorum communis was tested with the patient supine. The dorsiflexion of the lateral four toes was resisted with the examiner s fingers placed at the level of the proximal interphalangeal joint. 15 Triceps surae was first tested manually with the patient lying supine by assessing the strength of plantar flexion with the knee extended. Muscle strength was also assessed by asking the patient to rise on tiptoe on a single leg in the standing position, on one side at a time. Weakness was arbitrarily rated as very severe (2/5) when the patient was totally unable to rise and severe (3/5) when he could rise only 2 cm or less from the ground. This grading is functionally comparable with the standard grading because the complete inability to rise on tiptoe involves a similar dysfunction as the inability to dorsiflex the foot. The strength of quadriceps femoris was assessed with the patient in the prone position. The examiner held down the patient s thigh with his left hand, and resisted knee extension with the right hand. The prone position was chosen because it was easier for the examiner and more sensitive than other positions. 15 When a patient was unable to lie prone because of leg pain, the test was carried out in the supine position with the affected knee flexed at 40. The strength of the adductors was evaluated with the patient supine, resisting adduction of one thigh at a time. Gluteus maximus was tested with the patient prone, resisting hip extension with the knee extended. The hip flexors were tested with the patient in the sitting position. Study group. Of the 138 patients who had muscle weakness before surgery, nine were lost to follow-up, seven had died and six refused to attend for the long-term follow-up evaluation. This left 116 patients in the study, 66 men and 50 women, with a mean age of 49 years (17 to 83) at the time of surgery. Of these, 58 (50%) smoked and 13 (11%) drank more than half a litre of wine a day; 12 (10%) had arterial hypertension and 4 (3%) were diabetic. The latest follow-up was carried out at a mean of 6.4 years (3 to 10) after surgery. Before operation, 54 (47%) patients had weakness of a single muscle and 62 (53%) of two or more muscles. A mild loss of muscle strength was present in 78 patients (67%), a severe loss in 24 (21%) and a very severe loss in 14 (12%). The muscle most often showing loss of strength was EHL, followed by tibialis anterior, triceps surae, extensor digitorum communis (EDC), peronei, quadriceps femoris, gluteus maximus adductors and the hip flexors (Table I). The EHL was the muscle which most frequently showed a severe or very severe weakness; it was followed by the triceps surae, EDC, tibialis anterior, peronei, quadriceps femori and others. Of the patients with a very severe deficit lasting between eight days and 16 months, ten had involvement of the L5 nerve root, three of the S1 root, and one of both roots (Table II). All but two patients were operated on at a single lumbar level, at L2-L3 in three patients, L3-L4 in 16, L4-L5 in 63 and L5-S1 in 32. The two double-level discectomies were carried out at L3-L4 and L4-L5 in one patient and at L4-L5 and L5-S1 in the other. Most patients had an extruded or sequestrated herniation (Table III). The mean time interval Table I. Grade of preoperative muscle strength in the affected lower limb of 116 patients with herniation of the lumbar disc Muscle strength Total 4/5 3/5 2/5 or 1/5 Muscles involved Number % Number % Number % EHL 82 52 63 18 22 12 15 Tibialis anterior 51 37 72 11 21 3 6 Triceps surae 33 22 67 8 24 3 9 EDC 30 21 70 6 20 3 10 Peronei 17 13 76 3 18 1 6 Quadriceps femoris 14 12 85 2 14 Gluteus maximus 5 5 100 Adductors 3 3 100 Hip flexors 3 3 100 Total 238 168 70 48 20 22 9 VOL. 84-B, NO. 7, SEPTEMBER 2002

1042 F. POSTACCHINI, G. GIANNICOLA, G. CINOTTI Table II. Details of the 14 patients with very severe deficit of one or more muscles Preoperative Postoperative duration of Oswestry Age Site of deficit Muscle Muscle strength score Case (yrs) Gender herniation (days) involved* Preop Postop (%) 1 37 M L4-L5 75 EHL, ECD, TA 2,3,3 4,5,5 6 2 52 F L4-L5 15 EHL, TA, P 2,2,4 4,4,4 0 3 66 F L4-L5 20 EHL, TA 1,3 4,4 8 4 38 M L4-L5 480 EHL, TA, P 2,2,3 2,2,3 38 5 50 M L4-L5 50 EHL, TA, ECD 1,3,2 4,4,5 12 6 40 F L5-S1 45 EHL, TA, ECD 2,3,3 4,4,5 4 7 42 F L5-S1 15 TS 2 2 42 8 70 F L4-L5 25 EHL 2 5 10 9 55 M L4-L5 360 EHL, P 2,3 4,4 6 10 30 M L5-S1 25 TS, GM 2,4 2,5 69 11 41 F L4-L5 15 EHL, ECD 1,2 5,5 3 12 66 M L4-L5 21 EHL, TA, ECD 2,3,3 4,4,5 21 13 64 M L4-L5 60 TS, GM, TA, ECD, EHL 1,4,3,3,2 4,5,4,5,4 7 14 54 F L4-L5 8 EHL, TA, ECD, P 1,2,2,2 1,2,4,2 54 *ECD, extensor digitorum communis; TA, tibialis anterior; P, peronei; TS, triceps surae; GM, gluteus maximus the grades of preoperative and postoperative muscle strength are reported in sequence for each muscle involved Table III. Type of herniation related to the grade of motor deficit Preop muscle strength Postop muscle strength Type of 4/5 <4/5 5/5 <5/5 herniation Number % Number % Number % Number % Contained 35 45 5 13 36 50 4 14 Extruded 17 22 16 42 22 25 11 39 Sequestrated 16 20 14 37 21 24 9 32 Lateral 10 13 3 4 9 10 4 14 Total 78 38 88 28 between the onset of muscle weakness and surgery was 75 days (7 to 730) in the entire group, and 45 days (7 to 553) in the group with severe or very severe muscle weakness. After surgery, most patients with severe and all patients with very severe loss of muscle strength followed a protocol of neuromuscular rehabilitation, which included active exercises and electrical stimulation of the muscles which were involved. In some of the patients with very severe muscle weakness EMG was performed at various time intervals during the first few months after surgery. At the latest follow-up evaluation, the relevant data were recorded and the patients were asked to express their satisfaction with the result of surgery on a scale of 0 to 100. They also completed the Oswestry questionnaire to evaluate their functional disability 16 and were examined by an orthopaedic surgeon who was not involved in their treatment and who was blinded to the preoperative clinical data and the results of muscle assessments made at earlier follow-up examinations. Before the long-term follow-up evaluation was begun, the independent examiner assessed 50 patients with herniation of the lumbar disc not included in the study, together with the orthopaedic surgeon who had examined the patients under study before operation and at earlier follow-up studies. This was done in order to make the modalities of assessment and the criteria of grading muscle strength at the latest follow-up as similar as possible to the previous evaluations. Preoperative CT or MR scans were evaluated by an experienced neuroradiologist who was not involved in the preoperative diagnosis, in order to determine the size of the spinal canal, which was graded as normal, narrow but not stenotic or mildly stenotic. 17 Statistical analysis was by Pearson s test, Fisher s exact test and forward step-wise logistic regression. Results Complete recovery of muscle strength was found in 88 of the 116 patients (76%). Of the remainder, 18 (15%) had persistent weakness of a single muscle and ten (9%) of two or more muscles. Muscle strength was graded as 4 in all patients with involvement of lumbar roots (L2 to L5), except for the two who had a very severe deficit of the muscles supplied by the L5 root. One of these who had a long-standing deficit, obtained no improvement, and the other showed only a mild improvement in strength (cases 4 and 14, Table II). Of the three patients with involvement of the S1 root who preoperatively were unable to rise on tiptoe on the affected leg, two had no recovery of strength of the triceps surae (cases 7 and 10, Table II). Those patients who THE JOURNAL OF BONE AND JOINT SURGERY

RECOVERY OF MOTOR DEFICITS AFTER MICRODISCECTOMY FOR LUMBAR DISC HERNIATION 1043 Table IV. The timing of maximal recovery according to percentage of patients Grade of preoperative Follow-up deficit 2 mths 4 mths 6 mths Long-term Mild 60 83 96 100 Severe or very severe 33 48 84 100 showed partial or complete recovery, usually did so within two months of the operation; 83% of patients had maximal recovery of strength between two and four months after operation, while 4% ceased to improve at six months. Of the patients with severe or very severe motor deficits, 16% had further improvement between the six-month and the latest follow-up (Table IV). At the long-term follow-up, the muscles which most frequently remained weak, were the peronei and EHL, followed by the tibialis anterior, quadriceps femoris, triceps surae and EDC (Table V). A residual loss of muscle strength was observed in 16% of patients who had a mild weakness before operation and 39% of those with a severe or very severe muscle deficit. In the group of patients with mild preoperative weakness, the mean duration of the muscle deficit before surgery was significantly shorter in those who recovered completely (84 days) than in those who had a residual loss of strength (120 days) (p = 0.029). Likewise, in the group with severe weakness, there was a significant difference (p = 0.017) in the mean duration of the preoperative deficit between the patients who had a complete recovery of muscle strength (35 days, 7 to 90) and those with residual muscle weakness (69 days, 8 to 730). An inverse relationship was found between the severity of the preoperative muscle deficit and the ability to recover a complete motor function (p = 0.0046). A significant difference (p = 0.038) was found between the presence of an extruded or sequestrated herniation and the presence and severity of residual muscle weakness (Table III). No significant difference was found with regard to the mean age at the time of surgery between the patients who had a complete recovery (48 years) and those with partial recovery (51 years) of muscle strength. In the group with complete recovery there were 48 men (54%) and 40 women (45%), compared with 17 men (60%) and 11 women (39%) in the group with residual deficit, with no significant difference between the two groups. Smoking habits, intake of alcohol or comorbid diseases did not affect the recovery of muscle deficit. No significant difference was found in the nerve-root tension tests before surgery between the patients with complete and those with partial recovery of muscle strength. There was no relationship between the size of the spinal canal (normal, narrow or mildly stenotic) and the degree of recovery of muscle strength and there was no significant correlation between the persistence of muscle weakness and the persistence or recurrence of leg or low back pain. Of the eight patients who had a recurrent herniation demonstrated by MRI, five had complete and three had partial recovery after the initial surgery. The mean score reflecting patient satisfaction with the result of surgery was 89 in the group of patients with complete recovery and 78 in the group with some residual deficit; the difference was not statistically significant. The four patients with very severe deficit of the L5 or S1 nerve root who had no or very little recovery gave a mean score of 65. The mean score in the Oswestry questionnaire was slightly higher (15.3%) in the group of patients with a persistent mild deficit than in those who recovered completely (7.8%), but the difference was not significant. The patients with persistence of severe or very severe weakness of the large muscles supplied by the L5 or S1 nerve root, scored much higher (50.7%). Of the patients with a complete recovery, 88% returned to preoperative work and sport or leisure activities compared with 78% of those with a partial recovery. Again the difference was not significant. Discussion One of the reasons why the available data on the recovery of muscle strength after lumbar discectomy are conflicting may be that different systems for evaluating the muscle deficit have been used, and in no study are the clinical Table V. Postoperative muscle strength and rate of involvement Muscle strength Muscle 5/5 4/5 <4/5 involved Total Number % Number % Number % EHL 82 59 72 21 26 2 2 Tibialis anterior 51 40 78 9 18 2 4 Triceps surae 33 29 88 2 6 2 6 EDC* 30 29 97 1 3 Peronei 17 10 59 5 29 2 12 Quadriceps femoris 14 12 86 2 14 Gluteus maximus 5 5 100 Adductors 3 3 100 Hip flexors 3 3 100 Total 238 190 40 8 *extensor digitorum communis VOL. 84-B, NO. 7, SEPTEMBER 2002

1044 F. POSTACCHINI, G. GIANNICOLA, G. CINOTTI manoeuvres, used manually to test the muscle strength, described in detail. This is important because the measurements of power of EHL and tibialis anterior may vary considerably between the different modalities of manual assessment, whereas the strength of triceps surae can be adequately evaluated only by asking the patient to rise on tiptoe on the affected leg. Another reason may be the improper use of the terms foot-drop, paralysis and paresis. Davis 1 used the term foot-drop to indicate a complete or almost complete loss of dorsiflexion of the foot and toes, whereas Matsui et al 9 also applied the term to patients with moderate weakness of dorsiflexion. Some investigators have used the term paralysis for patients showing a marked, but not complete, loss of strength; 8 others have used the term paresis to include any grade of muscle weakness. 7,13 A limitation of our prospective study may be that except for a few patients, EMG was not undertaken before surgery or at the various follow-up evaluations. However, the clinical information obtained by manual testing is not inferior to that given by EMG, provided that the assessment of muscle strength is made using standard manoeuvres. Of the 138 patients included in our study, 22 (15%) were lost to follow-up. We believe this percentage of drop-out to be acceptable, considering the large number of patients enrolled and the length of follow-up, which ranged from three to 13 years. In our study, preoperative loss of muscle strength was found in 27% of patients. This figure is lower than the 50% incidence of motor deficit found by Hakelius, 13 but compares favourably with the incidence of 28% reported by Blaaw et al. 2 In previous investigations, motor deficits were found to be mild in most patients and very severe in 3% to 11% of those undergoing surgery for herniation of a lumbar disc. 1,6,9-12,18,19 After microdiscectomy, a full recovery of muscle strength was observed in 84% of our patients who had a mild preoperative deficit and 61% of those with a severe deficit. Of the patients with mild weakness, those who recovered had undergone surgery three months or less after the onset of muscle weakness, whereas all patients who underwent surgery later had no recovery. Similarly, of the patients with a severe deficit, those undergoing surgery within one month of the onset of weakness had a complete recovery, whereas most who underwent surgery after 70 days had an incomplete recovery of muscle strength. These findings suggest an inverse relationship between both the severity and the duration of preoperative muscle weakness and the ability to recover complete motor function. Such a correlation was not found in the group of patients with very severe loss of muscle strength. In these patients the occurrence and degree of recovery appear to be unpredictable. Some with deficits of short duration did not recover at all, whereas others with muscle weakness with a duration of a few months, did recover completely. The different behaviour of these patients may be related to a difference in the severity of compression of the nerve root or the abruptness with which it occurs. Experimental studies have shown that severe and rapid compression of spinal nerve roots produces more pronounced damage to the neural tissue than mild or slow compression, 20-23 and that extradural application of autologous nucleus pulposus induces structural and functional damage of varying severity in spinal nerve roots. 24 In our series, extruded and sequestrated herniations were more often associated with a severe or very severe preoperative deficit and a partial recovery after surgery. The peronei, EHL and tibialis anterior have the least capacity to recover normal strength. Triceps surae showed little tendency to recover when very severely impaired. Severe impairment of quadriceps femoris was rarely observed and all patients in whom this muscle was affected showed complete or almost complete recovery. Other factors were not found to relate significantly to the recovery of muscle function. In contrast to the findings in previous studies, 9-11 the severity of preoperative radicular pain and of nerve-root tension signs did not relate to the rate or degree of recovery in our patients. The same was true for gender, smoking habits, intake of alcohol and comorbid diseases and for the age of the patients at the time of surgery which, in two previous studies, 9,18 was found to be inversely related to the recovery of muscle strength. The patients satisfaction with the outcome of surgery and their capacity to return to work and leisure pursuits, corresponded to the degree of recovery of muscle strength and of the postoperative functional disability as evaluated by the Oswestry questionnaire. Nystrom 7 compared 56 patients undergoing microdiscectomy with 64 operated on by conventional techniques. He found a higher proportion of neurological recovery in the microdiscectomy group. In our investigation, the recovery of muscle strength compared favourably with studies which analysed patients submitted to conventional discectomy. 8,10,12,13,18 These studies, carried out before the advent of CT and MRI, used different diagnostic tools. It is, however, conceivable that the use of the operating microscope may have played an important role by enabling better visualisation for removal of disc fragments with minimal trauma. The results of our study indicate that in patients with herniation of a lumbar disc, which causes mild or severe weakness of muscles supplied by the L2 to L5 nerve roots, a complete or almost complete recovery of strength will occur after microdiscectomy. The chance of complete recovery is better the less severe the neural deficit is and the less delayed (beyond two months) the operation. Even if recovery is incomplete, the patient s subjective functional capacity is not significantly impaired. By contrast, in patients with a very severe deficit of L5 or S1 roots, surgery may lead to recovery of muscle strength, or alternatively, to no recovery and marked functional disability. In these patients, no factor emerges as predictive of whether a good or a poor recovery of neurological function will follow surgery. THE JOURNAL OF BONE AND JOINT SURGERY

RECOVERY OF MOTOR DEFICITS AFTER MICRODISCECTOMY FOR LUMBAR DISC HERNIATION 1045 No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. References 1. Davis RA. A long-term outcome analysis of 984 surgically treated herniated lumbar discs. J Neurosurg 1994;80:415-21. 2. Blaauw G, Braakman R, Gelpke GJ, Singh R. Changes in radicular function following low-back surgery. J Neurosurg 1988;69:649-52. 3. Weber H. Lumbar disc herniation: a controlled, prospective study with ten years of observation. Spine 1983;8:131-40. 4. Spannare BJ. Prolapsed lumbar intervertebral disc with partial or total occlusion of the spinal canal: a study of 30 patients with and 28 patients without cauda equina symptoms. Acta Neurochir (Wien) 1978;42:189-98. 5. Weir BKA. Prospective study of 100 lumbosacral discectomies. J Neurosurg 1979;50:283-9. 6. O Connel JEA. Protrusion of the lumbar intervertebral discs. J Bone Joint Surg [Br] 1951;33:8-30. 7. Nystrom B. Experience of microsurgical compared with conventional technique in lumbar disc operations. Acta Neurol Scand 1987;76:129-41. 8. Andersson H, Carlsson CA. Prognosis of operatively treated lumbar disc herniations causing foot extensor paralysis. Acta Chir Scand 1966;132:501-6. 9. Matsui H, Kanamori M, Kawaguchi Y, et al. Clinical and electrophysiologic characteristics of compressed lumbar nerve roots. Spine 1997;22:2100-5. 10. Knutsson B. How often do the neurological signs disappear after the operation of a herniated disc? Acta Orthop Scand 1962;32:352-56. 11. Jonsson B, Stromqvist B. Motor affliction of the L5 nerve root in lumbar nerve root compression syndromes. Spine 1995;18:2012-5. 12. Weber H. The effect of delayed disc surgery on muscular paresis. Acta Orthop Scand 1975;46:631-42. 13. Hakelius A. Prognosis in sciatica. Acta Orthop Scan Suppl 1970;129:6-76. 14. Seddon H. Surgical disorders of the peripheral nerves. Edinburgh and London: Churchill Livingstone. 1972:299. 15. Postacchini F, Gumina S. Clinical features. In: Postacchini F, ed. Lumbar disc herniation. Wien, etc; Springer-Verlag, 1999. 16. Fairbank JCT, Couper J, Davies JB, O Brien JP. The Oswestry low back pain disability questionnaire. Physiotherapy 1980;66:271-3. 17. Postacchini F. Management of lumbar spinal stenosis. J Bone Joint Surg [Br] 1996;78-B:154-64. 18. Naylor A. Late results of laminectomy for lumbar disc prolapse: a review after ten to twenty-five years. J Bone Joint Surg [Br] 1974;56-B:17-29. 19. Ebeling U, Reichenberg W, Reulen HJ. Results of microsurgical lumbar discectomy: review on 485 patients. Acta Neurochir Wein 1986;81:45-52. 20. Olmarker K, Rydevik B, Holm S. Edema formation in spinal nerve roots induced by experimental, graded compression: an experimental study on the pig cauda equina with special reference to differences in effects between rapid and slow onset of compression. Spine 1989;14:569-73. 21. Yoshizawa H, Kobayashi S, Morita T. Chronic nerve root compression: pathophysiologic mechanism of nerve root dysfunction. Spine 1995;20:397-407. 22. Olmarker K, Holm S, Rydevik B. Importance of compression onset rate for the degree of impairment of impulse propagation in experimental compression injury of the porcine cauda equina. Spine 1990;15:416-9. 23. Olmarker K, Rydevik B, Hansson T, Holm S. Compression-induced changes of the nutritional supply to the porcine cauda equina. J Spinal Disord 1990;3:25-9. 24. Olmarker K, Rydevik B, Nordborg C. Autologous nucleus pulposus induces neurophysiologic and histologic changes in porcine cauda equina nerve roots. Spine 1993;18:1425-32. VOL. 84-B, NO. 7, SEPTEMBER 2002