Anterior and posterior allografts in symptomatic thoracolumbar deformity.

The radiographic and clinical results of 105 patients with symptomatic spinal deformities were categorized retrospectively based on surgical approach and type of bone autograft or allograft used for each patient's fusion surgery into seven different groups and compared with one another. The three bone autograft control groups were posterior autograft only (n = 20), anterior autograft only (n = 6), and combined anterior and posterior autograft (n = 12). The allograft groups were posterior morcellized allograft (n = 7), posterior morcellized allograft and anterior autograft (n = 11), anterior structural interbody allografts and posterior mixture of allograft and autograft (n = 37), and anterior strut allograft with posterior mixture of allograft and autograft (n = 12). Radiographs revealed high pseudoarthrosis rates for adults with a posterior-only allograft and with anterior strut allografts spanning four or more levels. Results of the self-assessment outcomes questionnaire, at a mean follow-up period of 52 months, revealed less pain and improved cosmesis for all groups, and improved function in patients who had undergone combined anteroposterior fusion. The authors conclude that posterior cancellous allograft is a poor substitute for autograft bone and that strut allografts spanning more than four levels require technique modifications to enhance their effectiveness. In general, anterior structural allografts are effective in maintaining correction, result in fusion rates comparable to those of autografts, and correlate to improved outcomes.

Wrist position affects loading of the dorsal scaphoid: possible effect on extrinsic scaphoid blood flow.

Cadaver studies using radial artery injection techniques were used to study the vascular supply along the dorsal ridge of the scaphoid. These revealed an intraarticular membrane between the wrist capsule and the dorsal ridge of the scaphoid through which arteriolar vessels (25-100 microm internal diameter) passed. Biomechanical tests revealed that the extensor carpi radialis brevis may apply significant pressure to the dorsal ridge of the scaphoid when the wrist is flexed. The highest pressures occurred with the wrist flexed at 60 degrees or 90 degrees and in slight (15 degrees ) ulnar deviation. The authors suggest that these vascular and biomechanical factors may contribute to the aetiology for idiopathic osteonecrosis of the scaphoid.

The effect of spinal instrumentation on lumbar intradiscal pressure.

The purpose of this study was to investigate the effect of spinal instrumentation on the intradiscal pressure (IDP) within the fixed motion segment. In vitro biomechanical testing was performed in six single functional spinal units of fresh calf lumbar spines using a pressure needle transducer. Various loads were applied by a materials testing system device. In addition to intact spine (control), anterior spinal instrumentation (ASI) and pedicle screw fixation (PS) constructs, as well as destabilized spine were tested. Relative to the control, the destabilized spine tended to have an increased IDP; by 15% in axial compression and by 9-36% in flexion-extension. Compared to the control, PS decreased the IDP by 23% in axial loading and 51% in extension loading and increased it by 60% in flexion for each loading. ASI decreased the IDP by 32% in flexion and 1% in extension. Lateral bending produced symmetrical changes of IDP in the control and destabilized spine, but no change in the PS construct. The IDP of the ASI construct was decreased by 77% in ipsilateral bending and increased by 22% in contralateral bending. These results demonstrated that eccentric loading from the spinal instruments increased IDP and significant disc pressure may still exist despite an increase in motion segment stiffness after lumbar stabilization.

Lumbar fusion results related to diagnosis.

STUDY DESIGN: Pain outcome and functional outcome after primary lumbar fusion surgery were determined by a self-assessment questionnaire. The responses were correlated with various clinical parameters. OBJECTIVES: To determine the result of fusion surgery among patients in various diagnostic groups using semiquantitative outcome scales. SUMMARY OF BACKGROUND DATA: Most previous studies on the results of primary lumbar fusion have reported the presence of pain, but few have addressed function outcomes. Results of a literature review were inconclusive as to whether a patient's diagnosis is a predictor of improved results. METHODS: During the 3-year period from 1988 to 1990, 165 patients underwent a primary lumbar fusion procedure. They had a chart and radiograph review and were categorized into five major diagnostic groups: 1) pediatric, 2) grade I-II spondylolisthesis (low-slip), 3) grade III-IV spondylolisthesis (high-slip), 4) degenerative disc disease, and 5) postdiscectomy. At a follow-up period of 5 years (mean) after the fusion, patients were mailed a questionnaire in which they described their pain and functional status before and after their lumbar fusion surgery. Questionnaires were returned by 92% of the patients. The questionnaire scores, complications, and revision procedures were grouped by patient diagnosis and analyzed. RESULTS: Patient satisfaction with the results of primary lumbar fusion ranged from 69% (for the postdiscectomy group) to 100% (for the pediatric and high-slip groups). For all diagnostic groups, lumbar fusion resulted in a significant decrease in back pain and leg pain (visual analog scale), which was maintained throughout the follow-up period. For back pain, the pediatric and high-slip groups showed significantly more improvement than the degenerative disc disease or postdiscectomy groups. Leg pain among patients in the pediatric and high-slip groups was significantly more improved than leg pain among patients in the low-slip, degenerative disc disease, or postdiscectomy groups. There was no deterioration of pain scores during the follow-up period. After fusion, all groups had a significant decrease in Oswestry disability scores; patients in the pediatric and high-slip group had significantly more improvement than patients in the degenerative disc disease or postdiscectomy groups. High- and low-slip groups had a significant improvement in their pain drawing score. Medication use was substantially reduced in all groups. After fusion, a lack of improvement in back pain score or disability score was significantly correlated with pseudarthrosis. CONCLUSIONS: The outcome of primary lumbar fusion surgery was decreased pain and increased function for the majority of patients in all five diagnostic categories. The amount of improvement varied by diagnostic group. Patients with developmental conditions showed greater improvement than patients with degenerative conditions.

Lumbar intradiscal pressure after posterolateral fusion and pedicle screw fixation.

In vitro biomechanical testing was performed in single-functional spinal units of fresh calf lumbar spines, using pressure needle transducers to investigate the effect of posterolateral fusion (PLF) and pedicle screw constructs (PS) on intradiscal pressure (IDP), in order to elucidate the mechanical factors concerned with residual low back pain after PLF. IDP of 6 calf lumbar spines consisting of L4 and L5 vertebrae and an intervening disc was measured under axial compression, flexion-extension and lateral bending in the intact spine, PS, PLF and the destabilized spine. Relative to the intact spines, the destabilized spines showed increased IDP in all of lordings and moments. IDP under PS and PLF were significantly decreased in axial compression, extension and lateral bending loads (p<0.05). In flexion, IDP under PS and PLF increased linearly proportional to the magnitude of flexion moment and reached as high as IDP of the intact spines. These results demonstrated that despite an increase in the stiffness of motion segments after PLF and PS, significant high disc pressure is still generated in flexion. Flexibility of PS and PLF may cause increased axial load sharing of the disc in flexion and increased IDP. This high IDP may explain patients' persisting pain following PS and PLF.

Vertebral body MRI related to lumbar fusion results.

The evaluation of continued pain after a technically successful posterolateral lumbar spine fusion is often challenging. Although the intervertebral disc is often a source of low back pain, abnormal endplates may also be a focus of pain, and possibly a source of continued pain after a posterolateral fusion. MRI allows noninvasive evaluation for disc degeneration, as well as for abnormal endplates and adjacent vertebral body marrow. Previous studies have found inflammatory marrow changes, adjacent to abnormal endplates, associated with disc degeneration in low back pain patients. In this study, preoperative MRI scans in 89 posterolateral lumbar fusion patients were reviewed, by an independent radiologist, to determine whether vertebral body marrow changes adjacent to the endplates were related to continued pain. Independent chart review and follow-up telephone interview of all patients at a 4-year follow-up (mean) formed the basis for the clinical results. Vertebral body MRI signals consistent with inflammatory or fatty changes were found in 38% of patients, and always occurred adjacent to a degenerated disc. Inflammatory MRI vertebral body changes were significantly related to continued low back pain at P = 0.03. We conclude that posterolateral lumber fusion has a less predictable result for the subset of degenerative disc patients with abnormal endplates and associated marrow inflammation. More research is needed to determine the biological and biomechanical effects of posterolateral fusion upon the endplate within the fused segments. If indeed further study supports the hypothesis that abnormal endplates associated with inflammation are a source of pain, then treating the endplates directly by anterior fusion may be a preferred treatment for this subset of degenerative patients.

Revision of failed lumbar fusions. A comparison of anterior autograft and allograft.

STUDY DESIGN: The radiographic and clinical results of two different anterior structural grafts were compared in 38 patients who had combined anterior-posterior revision surgery for failed lumbar fusion. OBJECTIVES: Failed lumbar fusion surgery, such as pseudarthrosis or flatback deformity, may result in disabling pain. The optimum revision technique has yet to be defined. The authors of the current study sought to determine which of two different types of anterior graft yields the best results. SUMMARY OF BACKGROUND DATA: Posterior procedures for revision of a failed lumbar fusion have not yielded reliably successful results. A combined anterior-posterior approach may be effective in restoring sagittal balance and enhancing fusion rates. Recent studies have shown femoral ring allografts to be effective in lumbar fusion revision, but no studies have compared these with other types of structural grafts. METHODS: Thirty-eight patients with pseudarthrosis were treated with combined anterior-posterior lumbar spine fusion using either femoral ring allografts (26 patients) or tricortical iliac autografts (12 patients). Radiographic follow-up examination and retrospective patient self-assessment questionnaires were used to evaluate outcomes. Results were assessed by independent reviewers after a mean follow-up period of 35 months. RESULTS: Radiographic follow-up examination revealed acceptably low pseudarthrosis rates for structural autografts (0%) and allografts (6%). The questionnaires revealed significant improvement in pain for both groups. Allograft patients showed greater improvement in function, less pain medication usage, and higher overall success rates (83%) than autograft patients (64%). CONCLUSIONS: Femoral ring allografts are as effective, clinically and radiographically, as tricortical iliac autografts when used as an anterior structural element in revision lumbar spine fusion in patients who have undergone multiple surgical procedures for pseudarthrosis or flatback deformity. The slightly greater improvement for the allograft group needs to be confirmed in a larger study.

Determination of spinal facet joint loads from extra articular strains--a theoretical validation.

Loads on the facet joints of the lumbar spine may play a role in low back pain. Abnormal loading of the facets, either primarily or as a consequence of disc degeneration, may accelerate their degeneration. Study of these phenomena is difficult, since here are few methods for a direct measure of facet forces occurring in vivo. The authors developed a method for the direct measure of facet forces in a canine animal model of spine disease using strain gages. The method used empirical calibration of the strain gages, which was used to reduce experimental strain output to facet force during function. In this Technical Note, a theoretical model is formulated, solved and validated which gives a theoretical basis for the data reduction method. The facet is modeled as a cantilevered plate and deformations, as function of applied normal load, solved for using a finite difference method. The model is validated by comparison with two experiments with strain-gaged facets. Results from the model show that a minimum of three strain gages is required to determine uniquely the location and magnitude of an applied load to the facet, and that at least two gages should be placed in the region where the cranial articular process joins the pedicle with the gage axis parallel to that of the process. Plate surface strains were found to be insensitive to changes in the area of the applied loads, for a given resultant force magnitude. The method may be useful in other applications in which resultant force magnitude and location need to be measured on plate-like structures.

The use of bone allografts in the spine.

Bone allografts are commonly used in spinal surgery. Structural allografts placed anteriorly in the spine may be used as interbody grafts or as strut grafts spanning multiple segments. Posterior allografts are used to supplement autologous bone for spinal fusions in patients who lack sufficient host bone and to avoid significant donor site morbidity. In this paper the authors review their experience with allograft bone in spine surgery and the results reported in the literature. In the anterior cervical spine, interbody allografts have been used most successfully in single-level fusions. For thoracolumbar deformity, posterior allograft with instrumentation gives satisfactory results in pediatric patients but yields inferior results in adults unless it is combined with an anterior fusion. Fresh-frozen allograft bone has been shown to have higher fusion rates than freeze-dried allograft; ethylene oxide-sterilized allograft has shown uniformly poor results. Structural allografts have been useful for thoracolumbar deformity in both interbody and strut-grafting procedures. In the lumbar spine, allograft has a limited role in posterolateral fusion. For anterior interbody fusions, structural allografts, such as femoral ring allografts, have been used successfully to maintain intervertebral distraction, despite delayed incorporation. Successful use of allograft bone in the spine is dependent on the type of allograft bone used, its anatomic site of fusion, and the age of the patient.

Description and application of instrumented staples for measuring in vivo bone strain.

In vivo bone strain measurements using strain gages cemented to bony surfaces with cyanoacrylate polymers are limited in duration due to debonding of the gages from bone. As an alternative to the bone bonded strain gages, a technique was developed in which strain gages were first bonded to miniature staples and then the staples embedded into bone. The instrumented staples may be calibrated so that staple strain is directly proportional to bone strain. The method was first validated by comparing the staple output with cemented surface strain gages. Comparison of instrumented staples to cemented strain gages revealed only a 3% deviation from linearity during longitudinal bending; the staples were insensitive to transverse loading. The instrumented staples were then applied to the in vitro canine lumbar spine to determine L2-3 facet loads. Load testing, repeatibility of facet calibration, and validity testing of the in vitro instrumented staples were found to be comparable to that of the previous cemented strain gage techniques. In vivo facet joint application of the instrumented staples for periods of greater than 5 weeks gave load measurements comparable to our previous short-term in vivo studies obtained with cemented strain gages. The advantages of the instrumented staples are a more secure bonding to the bone, and less traumatic surgery for fixation.

Experimental measurement of ligament force, facet force, and segment motion in the human lumbar spine.

Facet forces, longitudinal ligament loads, and vertebral body motion were experimentally measured in five fresh human lumbar spine segments, L1-L2. Strain gages on the bone surface were used to quantify facet loads. Buckle transducers were used to measure anterior and posterior longitudinal ligament loads. The three-dimensional motion of the motion segment was measured with an instrumented spatial linkage. The facets were found to carry no load in flexion, large loads during extension (205 N at a 10 Nm moment and a 190 N axial load), torsion (65 N at a 10 Nm moment and a 150 N axial load), and lateral bending (78 N at a 3 Nm moment and a 160 N axial load). The facet contact site on the inferior articular process of L1 was found to move inferiorly to a position of tip impingement near the lamina as extension moments increased. Impingement occurred in the range of 4-6 Nm extension. The posterior and anterior longitudinal ligaments were predominantly loaded in flexion and extension, respectively. No ligament loads occurred in lateral bending and torsion. A 1 cm strip of the anterior longitudinal ligament carried loads up to 130 N at the largest extension moment of 11.4 Nm. The posterior longitudinal ligament had a 60 N load at the largest flexion moment of 7.1 Nm. There was no pre-load in the ligament detectable with the buckle transducers (> 4 N). The facets and ligaments began carrying load immediately with applied load, without a lax region. The experimental technique developed and used provides a good tool for obtaining simultaneous facet joint loads, ligament loads, and vertebral body motion without altering the motion segment.

In vivo analysis of canine intervertebral and facet motion.

Using an instrumented spatial linkage, a method for measuring intervertebral motion in vivo was developed and used on six dogs. The segmental motion was recorded as the animals were exercised in routine functions. The standing posture was found to be a repeatable position. During walking, the average excursion between opposing facets was 3.4 +/- 1.3 mm, as the L2-L3 motion segment moved into 2.3 degrees of kyphosis with respect to the standing position. This method has the ability of measuring facet motion (+/- 0.7 mm), vertebral body motion (+/- 0.5 mm), and vertebral body rotations (+/- 0.6 degrees) with suitable accuracy such that it is a useful tool in documenting the in vivo response of a motion segment to surgical procedures.

In vivo facet joint loading of the canine lumbar spine.

This study describes a technique to measure in vivo loads and the resultant load-contact locations in the facet joint of the canine lumbar spine. The technique is a modification of a previously described in vitro method that used calibrated surface strains of the lateral aspect of the right L3 cranial articular process. In the present study, strains were measured during various in vivo static and dynamic activities 3 days after strain gage implantation. The in vivo recording technique and its errors, which depend on the location of the applied facet loads, is described. The results of applying the technique to five dogs gave the following results. Relative resultant contact load locations on the facet tended to be in the central and caudal portion of the facet in extension activities, central and cranial in standing, and cranial and ventral in flexion or right-turning activities. Right-turning contact locations were ventral and cranial to left-turning locations. Resultant load locations at peak loading during walking were in the central region of the facet, whereas resultant load locations at minimum loading during walking were relatively craniad. This resultant load-contact location during a walk gait cycle typically migrated in an arc with a displacement of 4 mm from minimum to maximum loading. Static tests resulted in a range of facet loads of 0 N in flexion and lying to 185 N for two-legged standing erect, and stand resulted in facet loads of 26 +/- 15 N (mean +/- standard deviation [SD]). Dynamic tests resulted in peak facet loads ranging from 55 N while walking erect to 170 N for climbing up stairs. Maximum walk facet loads were 107 +/- 27 N. The technique is applicable to in vivo studies of a canine facet joint osteoarthritis model and may be useful for establishing an understanding of the biomechanics of low-back pain.

An experimental method for measuring force on the spinal facet joint: description and application of the method.

A technique is described for measuring load magnitude and resultant load contact location in the facet joint in response to applied loads and moments, and the technique applied to the canine lumbar spine motion segment. Due to the cantilever beam geometry of the cranial articular process, facet joint loads result in surface strains on the lateral aspect of the cranial articular process. Strains were quantified by four strain gages cemented to the bony surface of the process. Strain measured at any one gage depended on the loading site on the articular surface of the caudal facet and on the magnitude of the facet load. Determination of facet loads during in vitro motion segment testing required calibration of the strains to known loads of various magnitudes applied to multiple sites on the caudal facet. The technique is described in detail, including placement of the strain gages. There is good repeatability of strains to applied facet loads and the strains appear independent of load distribution area. Error in the technique depends on the location of the applied facet loads, but is only significant in nonphysiologic locations. The technique was validated by two independent methods in axial torsion. Application of the technique to five in vitro canine L2-3 motion segments testing resulted in facet loads (in newtons, N) of 74+ / -23 N (mean + / -STD) in 2 newton-meter, Nm, extension, to unloaded in flexion. Lateral bending resulted in loads in the right facet of 40+ / -32 N for 1 Nm right lateral bending and 54+ / -29 N for 1 Nm left lateral bending. 4 Nm Torsion with and without 100 N axial compression resulted in facet loads of 92+ / -27 N and 69+ / -19 N, respectively. The technique is applicable to dynamic and in vivo studies.

Facet loads in the canine lumbar spine before and after disc alteration.

The effect of externally applied spinal loads on facet mechanics before and after facet capsulectomy and disc alteration was investigated. Four control and four chymopapain-treated specimens were tested. Facet load and resultant load location were determined in the unaltered state, after facet capsulectomy, and after discectomy using a newly developed, noninvasive measurement technique. Resultant contact load locations shifted in a predictable manner. Caudal shift in contact locations occurred with chymopapain treatment or discectomy but was variable with capsulectomy. Facet load changed with applied load magnitude and direction; no significant change was noted after intradiscal chymopapain, discectomy, or capsulectomy. The significance of the lack of force change with disc narrowing is unknown. Perhaps, observed facet degeneration results from a change in the joint motion and contact location rather than change in facet load.

The effectiveness of fibrin glue sealant for reducing experimental pulmonary ari leak.

The effectiveness of fibrin glue as a biological sealant for pulmonary air leaks was determined in 16 dogs. A standardized pleural defect was made in the left lower lobe, and the quantity of air passing through a chest tube was assessed with a Collins respirometer. For the 8 randomly assigned control animals, the air leak decreased over 90 minutes from a mean of 1.4 L/min to a mean of 1.1 L/min (mean decrease, 19.8%). In the 8 randomly assigned fibrin glue-treated animals, the air leak decreased from a mean of 2.1 L/min to a mean of 0.5 L/min (mean decrease, 80.8%) (p less than 0.0001). Postoperative evaluation of survivors disclosed no increased adhesions in the glue-treated animals and complete resorption of the glue at 3 months. We conclude that in this animal model, fibrin glue reduced the size of pulmonary air leaks in the early period after thoracotomy and did not lead to increased intrapleural adhesions.

Speed of stress wave propagation in lung.

The speed of stress waves in the lung parenchyma was investigated to understand why, among all internal organs, the lung is the most easily injured when an animal is subjected to an impact loading. The speed of the sound is much less in the lung than that in other organs. To analyze the dynamic response of the lung to impact loading, it is necessary to know the speed of internal wave propagation. Excised lungs of the rabbit and the goat were impacted with water jet at dynamic pressure in the range of 7-35 kPa (1-5 psi) and surface velocity of 1-15 m/s. The stress wave was measured by pressure transducer. The distance between the point of impact and the sensor at another point on the far side of the lung and the transit time of the stress wave were measured. The wave speed in the goat lung was found to vary from 31.4 to 64.7 m/s when the transpulmonary pressure Pa-Ppl was varied from 0 to 20 cmH2O where Pa represents airway pressure and Ppl represents pleural pressure. In rabbit lung the wave speed varied from 16.5 to 36.9 m/s when Pa-Ppl was varied from 0 to 16 cmH2O. Using measured values of the bulk modulus, shear modulus, and density of the parenchyma, reasonable agreement between theoretical and experimental wave speeds were obtained.