Morphometric Study of the Nerve Roots Around the Lateral Mass for Posterior Foraminotomy

Objective

Morphometric data on dorsal cervical anatomy were examined in an effort to protect the nerve root near the lateral mass during posterior foraminotomy.

Methods

Using 25 adult formalin-fixed cadaveric cervical spines, measurements were taken at the lateral mass from C3 to C7 via a total laminectomy and a medial one-half facetectomy. The morphometric relationship between the nerve roots and structures of the lateral mass was investigated. Results from both genders were compared.

Results

Following the total laminectomy, from C3 to C7, the mean of the vertical distance from the medial point of the facet (MPF) of the lateral mass to the axilla of the root origin was 3.2-4.7 mm. The whole length of the exposed root had a mean of 4.2-5.8 mm. Following a medial one-half facetectomy, from C3 to C7, the mean of the vertical distance to the axilla of the root origin was 2.1-3.4 mm, based on the MPF. Mean vertical distances from the MPF to the medial point of the root that crossed the inferior margin of the intervertebral disc were 1.2-2.7 mm. The mean distance of the exposed root was 8.2-9.0 mm, and the mean angle between the dura and the nerve root was significantly different between males and females, at 53.4-68.4°.

Conclusion

These data will aid in reducing root injuries during posterior cervical foraminotomy.     

Vertebral Distraction during Anterior Cervical Discectomy and Fusion Causes Postoperative Neck Pain

Objective

Vertebral distraction is routinely performed during anterior cervical discectomy and fusion (ACDF). Overdistraction can injure the facet joints and may cause postoperative neck pain consequently. The purpose of this study was to investigate the clinical relevance of distraction force during ACDF.

Methods

This study included 24 consecutive patients with single level cervical disc disease undergoing single level ACDF. We measure the maximum torque just before the the arm of the Caspar retractor was suspended by the rachet mechanism by turning the lever on the movable arm using a torque meter. In order to turn the lever using the torque driver, we made a linear groove on the top of the lever. We compared the neck disability index (NDI) and visual analogue scale (VAS) scores between the high torque group (distraction force>6 kgf·cm) and the low torque group (distraction force≤6 kgf·cm) at routine postoperative intervals of 1, 3, 5 days and 1, 3, 6 months.

Results

The VAS scores for posterior neck pain had a linear correlation with torque at postoperative 1st and 3rd days (y=0.99×-1.1, r²=0.82; y=0.77×-0.63, r²=0.73, respectively). VAS scores for posterior neck pain were lower in the low torque group than in the high torque group on both 1 and 3 days postoperatively (3.1±1.3, 2.6±1.0 compared with 6.0±0.6, 4.9±0.8, p<0.01). However, the difference in NDI scores was not statistically significant in all postoperative periods.

Conclusion

Vertebral distraction may cause posterior neck pain in the immediate postoperative days. We recommend not to distract the intervertebral disc space excessively with a force of more than 6.0 kgf·cm.     

Difference in canal encroachment by the fusion mass between anterior cervical discectomy and fusion with bone autograft and anterior plating,and stand-alone cage

We conducted a prospective randomized study comparing stand-alone cage and bone autograft and plate implants in anterior cervical discectomy and fusion (www.clinicaltrials.gov, NCT01011569). Our interim analysis showed autologous bone graft with plating was superior to a stand-alone cage for segmental lordosis. During this analysis, we noted a difference in canal encroachment by the fusion mass between the two fusion groups. A narrow cervical spinal canal is an important factor in the development of cervical spondylotic myelopathy, therefore this unexpected potential risk of spinal cord compression necessitated another interim analysis to investigate whether there was a difference in canal encroachment by the fusion mass between the two groups. Patients had a minimum 1 year of follow-up. The Neck Disability Index, neck and arm pain Visual Analog Scales and lateral radiographs, including bone fusion patterns, were evaluated. Twenty-seven (16 males, 11 females, mean age 54.8 years) and 31 (24 males, seven females, mean age 54.5 years) patients were in the cage and plate group, respectively. Both groups improved after surgery. Fusion began at 2.6 months and 1.3 months and finished at 6.7 months and 4.0 months in 24 (88.9%) and 28 (90.3%) patients in the cage and plate group, respectively. Encroachment into the spinal canal by the fusion mass was significantly different between the fusion types, occuring in 21 (77.8%) patients in the cage group versus six (19.4%) in the plate group (p = 0.003). There was a high incidence of spinal canal encroachment by the fusion mass in the stand-alone cage group, possibly limiting use in narrow spinal canals.     

Free Hand Pedicle Screw Placement in the Thoracic Spine without Any Radiographic Guidance : Technical Note, a Cadaveric Study

Thoracic pedicle screw fixation techniques are still controversial for thoracic deformities because of possible complications including neurologic deficit. Methods to aid the surgeon in appropriate screw placement have included the use of intraoperative fluoroscopy and/or radiography as well as image-guided techniques. We describe our technique for free hand pedicle screw placement in the thoracic spine without any radiographic guidance and present the results of pedicle screw placement analyzed by computed tomographic scan in two human cadavers. This free hand technique of thoracic pedicle screw placement performed in a step-wise, consistent, and compulsive manner is an accurate, reliable, and safe method of insertion to treat a variety of spinal disorders, including spinal deformity.     

Comparison between Anterior Cervical Decompression with Fusion and Posterior Cervical Fusion with Wide Facetectomy for Treatment of Severe Bony Foraminal Stenosis

ObjectiveTo compare the anterior cervical discectomy and fusion (ACDF) and posterior cervical fusion (PCF) with wide facetectomy in the treatment of parallel-shaped bony foraminal stenosis (FS). MethodsThirty-six patients underwent surgery due to one-or-two levels of parallel-shaped cervical FS. ACDF was performed in 16 patients, and PCF using CPS was performed in 20 patients. All patients were followed up at 1, 3, 6, and 12 months postoperatively. Standardized outcome measures such as Numeric rating scale (NRS) score for arm/neck pain and Neck disability index (NDI) were evaluated. Cervical radiographs were used to compare the C2–7 Cobb’s angle, segmental angle, and fusion rates. ResultsThere was an improvement in NRS scores after both approaches for radicular arm pain (mean change -6.78 vs. -8.14, p=0.012), neck pain (mean change -1.67 vs. -4.36, p=0.038), and NDI score (-19.69 vs. -18.15, p=0.794). The segmental angle improvement was greater in the ACDF group than in the posterior group (9.4°±2.7° vs. 3.3°±5.1°, p=0.004). However, there was no significant difference in C2–7 Cobb angle between groups (16.2°±7.9° vs. 14.8°±8.5°, p=0.142). As a complication, dysphagia was observed in one case of the ACDF group. ConclusionIn the treatment of parallel-shaped bony FS up to two surgical levels, segmental angle improvement was more favorable in patients who underwent ACDF. However, PCF with wide facetectomy using CPS should be considered as an alternative treatment option in cases where the anterior approach is burdensome.     

Changes in the axial orientation of the zygapophyseal joint in the subaxial cervical spine from childhood to middle-age,and the biomechanical implications of these changes

The objective of this study was to investigate the effect of age on facet orientation (FO) of the cervical spine during development, maturation, and degeneration. Computed tomography (CT) data of the cervical spine of 131 subjects without pathology of the cervical spine were analyzed. Subjects were categorized as: pediatric (n = 36, 8–16 years old), young adult (n = 33, 18–24 years old), and middle-age (n = 62, 40–59 years old). Serial CT scans were reconstructed by image processing. The FO in the axial plane was measured bilaterally at each vertebral level from C3/4 to C6/7. Differences in FO were analyzed between the 3 groups. The degree of external rotation of FO significantly decreased at C3/4 and C4/5 with increasing in age, and maximum external rotation was observed at C5/6. The external rotation at C6/7 increased from pediatric to young adulthood, but decreased from young adults to middle-aged adults. The dominant external rotation was seen in C4/5 and C5/6 in the pediatric age group, C5/6 and C6/7 in young adults, and C4/5 and C5/6 in middle-aged adults. These results lead us to conclude that FO in the axial plane exhibits significant differences with age. The degree of external rotation with respect to FO at each vertebral level is comparable to changes in cervical spinal dynamics with age. Hence, FO in the axial plane is a biomechanical parameter that can be used to assess changes in the cervical spinal during maturation and degeneration.     

Anatomical Morphometric Study of the Cervical Uncinate Process and Surrounding Structures

Objective

The purpose of this study is to elucidate the anatomic relationships between the uncinate process and surrounding neurovascular structures to prevent possible complications in anterior cervical surgery.

Methods

Twenty-eight formalin-fixed cervical spines were removed from adult cadavers and were studied. The authors investigated the morphometric relationships between the uncinate process, vertebral artery and adjacent nerve roots.

Results

The height of the uncinate process was 5.6-7.5 mm and the width was 5.8-8.0 mm. The angle between the posterior tip of the uncinate process and vertebral artery was 32.2-42.4°. The distance from the upper tip of the uncinate process to the vertebral body immediately above was 2.1-3.3 mm, and this distance was narrowest at the fifth cervical vertebrae. The distance from the posterior tip of the uncinate process to the nerve root was 1.3-2.0 mm. The distance from the uncinate process to the vertebral artery was measured at three different points of the uncinate process : upper-posterior tip, lateral wall and the most antero-medial point of the uncinate process, and the distances were 3.6-6.1 mm, 1.7-2.8 mm, and 4.2-5.7 mm, respectively. The distance from the uncinate process tip to the vertebral artery and the angle between the uncinate process tip and vertebral artery were significantly different between the right and left side.

Conclusion

These data provide guidelines for anterior cervical surgery, and will aid in reducing neurovascular injury during anterior cervical surgery, especially in anterior microforaminotomy.     

Surgical management of primary tumors of the cervical spine: surgical considerations and avoidance of complications

Abstract

Objective:

Primary cervical spinal tumors are rare tumors of the spine and are associated with significant morbidity and mortality. Such tumors include multiple myeloma, chordomas, giant cell tumors, hemangiomas, osteosarcomas, chondrosarcomas, synovial sarcomas, aneurysmal bone cysts (ABC), hemangiomas, eosinophilic granulomas, osteoid osteomas, and osteoblastomas. We review the surgical decision-making process and identify critical key steps for surgical complication avoidance. We also present case illustrations demonstrating such pathological diagnoses and surgical treatments performed.

Methods:

We retrospectively review the literature regarding the most common primary cervical spinal tumors that have undergone surgical resection with or without adjuvant treatment.

Results:

En bloc resection of primary cervical tumors resulted in significantly increased progression-free survival and overall survival. From the limited data, adjuvant treatment with proton-beam therapy for chordomas has potential benefit. Neo-adjuvant chemotherapy for Ewing’s sarcoma and osteogenic sarcoma has shown some promise, with en bloc resection demonstrating stronger benefit for osteogenic sarcoma.

Discussion:

En bloc resection for primary spinal tumors has proven to be the standard of care in spinal oncology. Adjuvant and neo-adjuvant treatments such as chemotherapy and radiotherapy variants (conventional, proton beam, cyberknife) need to be studied further in most primary tumor types to become standard of care. Chordoma management is more widely studied with en bloc resection and adjuvant proton-beam therapy demonstrating improved progression-free survival and overall survival. Surgical management and adjuvant treatment strategies are case dependent, depending on tumor histology, patient neurological examination, prior surgeries at that level, and prior adjuvant treatment.     

地震中颈椎骨折伴颈脊髓损伤和(或)肺部并发症患者救治体会

目的 分析四川大地震中颈椎骨折伴颈脊髓损伤患者肺部并发症的发生原因并总结救治体会.方法 回顾性分析泸州医学院附属医院神经外科自2008年5月12日至2008年8月6日收治的9例颈椎骨折伴颈脊髓损伤患者的临床资料.结果 颈椎骨折伴颈脊髓损伤患者均采取手术治疗,其中6例伤后5 d内发生肺部并发症,其中肺炎3例,通气障碍2例,肺水肿和血气胸1例,经积极的呼吸道管理等非手术治疗,肺部并发症都得到有效控制.结论 颈椎骨折伴颈脊髓损伤急性期(<5 d)常发生严重肺部并发症.以高位损伤(C4>以上)、合并胸部损伤、高龄、有肺部疾患史与吸烟史者更易发生,早期发现后内科治疗效果好.     

Pedicular and Extrapedicular Morphometric Analysis in the Korean Population : Computed Tomographic Assessment Relevance to Pedicle and Extrapedicle Screw Fixation in the Thoracic Spine

Objective

To evaluate the anatomical parameters that must be considered when performing thoracic transpedicular or extrapedicular screw fixation.

Methods

We selected 958 vertebrae (1,916 pedicles) from 98 patients for analysis. Eight parameters were measured from CT scans : the transverse outer pedicular diameter, transverse inner pedicular diameter, length, angle, chord length of the pedicles and the transverse width, angle, and chord length of the pedicle-rib units.

Results

The age of the patients ranged from 21 to 82 years (mean : 48.2 years) and there were 57 men and 41 women. The narrowest transverse outer pedicular diameter was at T5 (4.4 mm). The narrowest pedicle length was at T1 (15.9 mm). For pedicle angle, T1 was 31.6 degrees, which was the most convergent angle, and it showed the tendency of the lower the level, the lesser the convergent angle. The chord length showed a horizontal pattern with similar values at all levels. For the PRU width, T5 showed a similar pattern to the pedicle width at 13.4 mm. For the PRU angle, T1 was the largest angle at 46.2 degrees and the tendency was the lower the level, the narrower the angle. For chord length, T1 was the shortest at 46.9 mm and T8 was the longest at 60.1 mm.

Conclusion

When transpedicular screw fixations carried out at the mid-thoracic level, special care must be taken because there is a high chance of danger of medial wall violation. In these circumstances, extrapedicular screw fixation may be considered as an alternative treatment.     

Space Available for Cord,Motion, and disc degeneration at the adjacent segments level of degenerative cervical spondylolisthesis using kinematic MRI

The objective was to evaluate motion, disc degeneration and Space Available for Cord (SAC) at the adjacent segments of degenerative cervical spondylolisthesis (DCS) using kinematic Magnetic Resonance Imaging (kMRI). The cervical spine kMRI of sixty-one DCS spinal levels (38 anterolisthesis and 23 retrolisthesis) were analyzed at the listhesis level and its adjacent segments in three position using kMRI. MRAnalyzer3 was used to analyze translation, angular motion and SAC. The caudad level had significantly less translation motion than the listhesis level in overall DCS or grade 2 anterolisthesis group (P < 0.05). The cephalad level had significant more translational motion than the caudad level in overall DCS or grade 1 retrolisthesis group (P < 0.05). For disc degeneration, the cephalad level had the least disc degeneration and showed significant difference with the listhesis level in overall DCS or overall anterolisthesis or overall retrolisthesis or grade 2 anterolisthesis or grade 1 or 2 retrolisthesis (P < 0.05). For SAC, the listhesis level had the narrowest space in overall DCS groups. In neutral position, grade 1 anterolisthesis had significantly larger SAC at the listhesis and the cephalad level than grade 1 retrolisthesis (P < 0.05). In conclusion, DCS affected cervical spine motion and kinematics. Grade 1 retrolisthesis showed tendency of narrower SAC at the cephalad level more than the same grade anterolisthesis. The likelihood of the adjacent segment disease and spinal cord compression are higher in both grade 2 anterolisthesis and retrolisthesis.     

Ulnar neuropathy at the elbow: Reappraisal of the wrist-upper arm latency difference between ulnar and median nerves

ObjectivesTo evaluate the sensitivity and specificity of the latency difference (DLat) between ulnar and median nerves of the arm after stimulation at the wrist; one of the easiest techniques proposed for recognizing ulnar neuropathy at the elbow (UNE). As latency difference is not a standardized technique, we set up a multicenter study to recruit large numbers of normal subjects and patients with UNE or generalized neuropathy.MethodsSix centers participated in the study with data obtained from three groups of participants, controls (CTRLs), patients with UNE and patients with generalized neuropathy (GNP).We first verified the anatomical superposition of the ulnar and median nerves in cadaver examination. The optimal recording site for these two nerves was found to be 10 cm above the medial epicondyle. We then standardized the position of the arm with full extension of the elbow and stimulated first the median and then the ulnar nerves at the wrist. CTRLs were examined on both arms at two consecutive visits.ResultsWe recorded 32 idiopathic UNE cases, 44 GNP patients and 62 controls.We demonstrated that a DLat cut-off value of 0.69 ms brings a sensitivity of 0.86 and specificity of 0.89 to discriminate CTRLs from UNE. We also validated that intra-examiner reproducibility was good.ConclusionWe report a lower normal value for DLat than reported in several non-standardized studies and CTRL and UNE groups have clearly separated DLat values.SignificanceDue to its high sensitivity, our standardized technique could be used as a first-line diagnostic tool when UNE is suspected.     

The misdiagnosis of acute cervical spine injuries and fractures in infants and children: the 12-year experience of a level I pediatric and adult trauma center

Objective The objective was to determine the frequency of acute cervical spine injuries and fractures that were misdiagnosed in infants and children (14 years) initially evaluated at a pediatric and adult urban level I trauma center.Methods and results This was a retrospective, single-institution, case series of pediatric cervical spine injuries and fractures that were misdiagnosed during initial emergency room imaging evaluation. Misdiagnosed cases were those cases whose imaging studies initially obtained in the emergency room were misinterpreted based on reevaluation by a senior trauma radiologist blinded to the initial results. Nineteen percent (7 out of 37) were misdiagnosed on initial emergency room imaging evaluation. Five percent were true missed fractures, and 14% were normal and/or developmental variants read as fractures or dislocation.Conclusions The error rate for infants and children 8 years was 24%, and for children 9 years was 15%. The occiput to C2 region was the most common site of diagnostic error. The most common factors predisposing to misdiagnosis were unfamiliarity with pediatric cervical spine anatomy, failure to recognize normal variants seen during growth and development, and suboptimal conventional film techniques.     

The development of the relationship between dorsal root afferents and motoneurons in the larval bullfrog spinal cord

The relationship of dorsal root afferents to motoneuron somata and dendrites was studied by labelling dorsal and ventral roots of the tadpole lumbar enlargement with HRP at different stages of hindlimb development. Procedures were used which allowed for sequential light and electron microscopic analysis to determine whether close appositions between labelled elements represented synaptic contacts. Lateral motor column (LMC) motoneuron dendrites grow first into the lateral funiculus, and later begin arborizing within the spinal gray, concurrent with the arrival of developing dorsal root afferent fibers. Mature-appearing synaptic contacts between dorsal root afferents and motoneuron dendrites are established first on distal dendrites, and are observed on progressively more proximal dendrites as hindlimb development proceeds. Migrating motoneurons were also labelled in some animals. Distinct dorsal and ventral migratory pathways were noted; cells migrating dorsally were contacted by developing dorsal root afferents. Migrating motoneurons were associated with radially oriented processes, and were often closely apposed to other cells. The coincident development of dorsal root projections and the motoneuron dendrites which these fibers innervate in the adult, as well as the interaction between these two systems during cell migration, suggest that these two systems may be interdependent in establishing their normal relationship during development.     

神经生长因子的脑保护作用与Caspase-3表达的相关性

目的 研究神经生长因子(NGF)的脑保护时间窗与半胱天冬酶-3(Caspase-3)表达的相关性.方法 采用兔局灶性脑缺血再灌注损伤模型,分别于再灌注后0h、1h、3h和6h将NGF立体定向导入梗死灶周,再灌注72h观察神经功能、梗死体积、灶周凋亡率和Caspase-3表达.结果 再灌注0h、1h和3 h灶周给予NGF,梗死体积分别较对照组下降50.1%、42.5%和35.2%,相应的灶周凋亡率及Caspase-3表达明显下降,神经功能恢复较好,用药越早越明显;再灌注6h给药,则无明显作用.相关分析显示梗死体积变化与Caspase-3表达具有明显相关性(P＜0.05).结论 NGF脑保护治疗时间窗与Caspase-3表达相关,抑制Caspase-3表达可能是NGF介导神经保护作用的机制之一.     

Relationship between the states of spinal impact injuries and magnetically evoked EMGs in rats

Abstract

The relationship between the states of spinal Impact injuries and magnetically evoked electromyograms (EMGs) were studied in rats. Impact injuries to the spinal cord were induced at a depth of 0.25-2.0 mm by insertion of a cylinder tip measuring 2 mm in diameter into the lumbar vertebrae L1-L2. Magnetically induced electromyograms for the brain and lumbar vertebrae L4-L5 were recorded from the tibialis anticus and the gastrocnemius muscles. H-reflex was not induced by the spinal cord injury (SCI) at a depth of 0.25 mm, although motor evoked potential (MEP) was observed. Continuous waves following the M- response were observed in the SCI at a depth of 0.25 mm. Elevation of the threshold, reduction of its latency and decrease in amplitude of the M-responses were observed at an injury depth of 0.5 mm or deeper. With SCI magnitude from mild (0.5 mm depth) to severe (1.0 mm depth), the amplitudes of the M-response were decreased, and the latency of the M-response was shorter than that of the control. The F-response was accelerated in severe SCI. Our results indicated that there was a relationship between extensive injury legions and the H-reflex F- and M-responses in magtnetically evoked EMGs. Magnetically evoked EMGs are useful for monitoring the states of SCI. [Neurol Res 2000; 22: 727-732]     

兔脊髓损伤后脊髓运动诱发电位D波与后肢运动功能的关系

目的 探讨兔脊髓损伤模型损伤后双后肢肌力和脊髓运动诱发电位D波之间的关系.方法 45只兔按随机数字表法分为打击组(显露硬脊膜并打击,并按打击能量大小分为50gcf组、75 gcf组、100gcf组、125 gcf组、150gcf组、175 gcf组、200 gcf组、250 gcf组)和对照组(显露硬脊膜但不打击).打击后和第4周末记录兔双后肢肌力、运动诱发电位D波的潜伏期和波幅,并在第4周末取家兔脊髓行神经微丝(NF)免疫组化染色及病理形态观察,测量NF吸光度值.结果 脊髓损伤当时和第4周末脊髓硬膜外运动诱发电位D波波幅下降的幅度与兔后肢运动功能呈线性相关关系(R2=0.98,P＜0.001,回归曲线Y=0.33+4.82X;R2=0.99,P＜0.001,回归曲线Y=0.04+0.51X),而D波潜伏期的变化则与术后肌力的变化无相关关系(R2=-0097,P=0.573;R2=-0.165,P=0.337);NF吸光度值随着D波波幅的下降而下降,两者呈线性相关关系(R2=0.87,P＜0.001,回归曲线Y=0.12+0.58X).结论 在脊髓损伤中可以通过脊髓硬膜外运动诱发电位D波波幅的下降程度作为判断脊髓损伤运动功能障碍程度的指标.

Abstract：

Objective To study the relationship between D-wave of thc spinal motor evoked potential and hindlimb motor function status (myodynamia) of rabbits with experimental spinal cord injury. Methods Forty-five rabbits were randomly divided into a control group and 8 injured groups (receiving Allen's injury of50, 75, 100, 125, 150, 175, 200 and 250 gcf, respectively). The myodynamia of hindlimbs and the latency and amplitude of D-wave of the spinal motor evoked potential were recorded right after the injury and in the 4th week of injury, respectively. Simultaneously in the 4th week of injury,the rabbits' spinal sections were stained with neurofilament (NF) immunohistochemistry and their pathological morphologies were observed; and optical densities of NF were measured. Statistic analysis of correlation between myodynamia of hindlimbs and latency and amplitude of D-wave was performed.Results A statistic linear correlation was found between amplitudes declination of D-wave of the spinal motor evoked potential and modified Tarlov's scores right after the injury and in the 4th week of injury (R2=0.98, P＜0.00, regression curve: Y=0.33+4.82X; R=0.99, P＜0.001, regression curve: Y=0.04+0.5 1X),but no statistical correlation was noted between myodynamia and the latency of D-wave of the spinal motor evoked potential at the same time periods(R2=-0.097, P=0.573; R2=-0.165, P=0.337). And the optical density of NF decreased following the amplitude declination of D-wave of the spinal motor evoked potential with linear correlation (R2=0.87, P＜0.001, regression curve: Y= 0.12+0.58X).Conclusion Amplitude declination of D-wave of the spinal motor evoked potential could be used to evaluate the severity of dyskinesia in the injured spinal cord.     

Relations between axon length and axon caliber: Is maximum conduction velocity the factor controlling the evolution of nerve structure?

A search was made for any existent relationship between the length of a nerve fiber and the caliber of its axon. This was done in the hope of defining morphological parameters useful for assessing conduction time.Four fiber populations were examined: (1) phrenic fibers in rat and rabbit during different phases of body growth; (2) phrenic fibers of mature animals of greatly different body size including mouse and cow; (3) rat intercostal nerves which vary in length by a factor exceeding 5 due to the funnel-shape of the thorax; and (4) ventral root fibers of the cow.In all of these fiber populations, there was no evidence for a direct relationship between the length of a fiber and its caliber. Rather, a tendency was noted for fiber caliber to approach certain ceilings independent of length.These data, seen in conjunction with other information on fiber structure, cast serious doubt on the widely accepted concept that maximum conduction velocity is the factor controlling nerve structure. A much more likely factor controlling the structure of myelinated nerve fibers is the capacity to modulate information by frequency coding of impulses.