Residual Bone Fragments Research Articles

Foraminoplasty Performed with a Trephine and a New Tool in Transforaminal Endoscopic Lumbar Discectomy: A Single-Center Retrospective Study.

Foraminoplasty is an important step in transforaminal endoscopic lumbar discectomy (TELD). A trephine is widely used in foraminoplasty. However, foraminoplasty using a trephine alone sometimes fails to remove the resected bone, resulting in the bone remaining in the foramen or spinal canal, which can potentially cause neurological irritation or injury. The objective of this study is to introduce a self-designed tool, referred to as an anchoring drill, for use with a trephine in foraminoplasty in TELD and to evaluate its advantages. A retrospective review was performed to identify patients who underwent L4-5 TELD between January 2019 to January 2022. Foraminoplasty was performed in all patients. Depending on whether the anchoring drill was used or not, patients were divided into two groups. Surgery-related parameters and complications were reviewed. Visual analog scale (VAS) and Japanese Orthopaedic Association (JOA) scores were also assessed for all patients. SPSS statistical software was used for statistical calculation. A total of 100 patients were included (55 in the anchoring drill group and 45 in the trephine group). The incidence of residual bone fragments after foraminoplasty of the anchoring drill group was 9.09%, which was lower than that of the trephine group, at 33.33% (p < 0.05). The mean endoscopic operation time of the anchoring drill group was shorter than that of the trephine group (p < 0.05). The mean fluoroscopy time and duration of foraminoplasty showed no significant differences between the two cohorts. The total perioperative complication incidence was lower in the anchoring drill group, in which the neural irritation incidence showed a significant difference (anchoring drill group: 3.64%, trephine group: 17.78%, p < 0.05). VAS and JOA scores were significantly improved after the operation for all patients (p < 0.001), however, no statistical differences were found between the two groups at each follow-up visit. The combination of a trephine with an anchor drill was demonstrated to be safe and effective in foraminoplasty in TELD, improving the success rate of foraminoplasty and reducing neurological complications compared to using trephine alone.

Unilateral biportal endoscopic technique combined with percutaneous transpedicular screw fixation for thoracolumbar burst fractures with neurological symptoms: technical note and preliminary report

BackgroundPrevious studies on thoracolumbar fractures with neurological symptoms have focused on how to achieve satisfactory fracture reduction, adequate nerve decompression, and stable spinal alignment. With the development of the minimally invasive spine surgery technique, achieving satisfactory treatment results and reducing iatrogenic trauma at the same time has become a new goal of spinal surgery. This research used percutaneous transpedicular screw distraction to partially reduce the fractured vertebrae, followed by completing nerve decompression and reducing residual displacement bone fragments with the assistance of the unilateral biportal endoscopic (UBE) technique to achieve full protection of bone-ligament tissue and obtain good clinical efficacy.MethodsGuide wires were safely inserted into the fractured vertebra and adjacent upper and lower vertebra under the surveillance of anteroposterior and lateral X-ray fluoroscopy. Transpedicular screws were implanted via guide wires on the side with mild neurological deficits or bone fragment compression (the opposite side of the endoscopic operation). A titanium rod was installed and moderately distracted to reduce the fractured vertebra. Then, under the guidance of the endoscopic view, the laminectomy and ligamentum flavum resection were completed according to the position of the protruding bone fragment into the spinal canal, and the compressed dural sac or nerve root was fully exposed and decompressed. An L-shaped replacer was used to reduce residual bone fragments. The ipsilateral transpedicular screws and rod were installed and adjusted to match the contralateral side. The drainage tube was indwelled, and the incision was closed. The preoperative and postoperative images of the patients were evaluated, and the recovery of neurological symptoms was observed.ResultsSurgery was successfully completed on all six patients, and no intraoperative conversion to open surgery was performed. Postoperative images showed good reduction of the protruding bone fragment and good placement of all screws. At the last follow-up, the neurological symptoms of all patients returned to normal.ConclusionThe UBE technique combined with percutaneous transpedicular screw fixation in the treatment of thoracolumbar fractures with neurological symptoms can effectively achieve the reduction of displaced bone fragments, improve damaged nerve function, stabilize spinal alignment, and protect the integrity of bone-ligament tissue.

Application of a universal system of epitesis fixation in the rehabilitation of patients with maxillofacial defects

The aim of the study was the creation of a universal epithesis fixation system that increases the reliability of its functioning in prosthetics of patients with maxillofacial defects of various genesis. After the surgical preparation of patients, in accordance with the indications of the underlying disease, epithesis was made from rigid silicone elastomers using CAD/CAM technologies. For the production of attachments, the VT1-0 titanium alloy common in medicine was used. A universal epithesis fixation system was used for prosthetics of various parts of the patient's face. Depending on the clinical situation and the size of the prosthetics area, an intermediate lattice plate (mesostructure) of the appropriate size and configuration was selected. Two types of attachments were used to fix epithets: magnetic with a counterpart and non-magnetic mushroom-shaped. The universal epithesis attachment system is distinguished by the possibility of using each hole of the lattice, both for attaching it to the bone structure and for installing attachments, which makes it easy to fix the lattice to residual bone fragments and center the position of the transition elements and attachments depending on the aesthetic and functional needs of the structure in whole. The success of prosthetics largely depends on the epithesis attachment system. The proposed universal epithesis fixation system allows its use in various clinical situations and also reduces the cost of prosthetics for patients with maxillofacial defects.

Ablation of Proliferating Osteoblast Lineage Cells After Fracture Leads to Atrophic Nonunion in a Mouse Model.

Nonunion is defined as the permanent failure of a fractured bone to heal, often necessitating surgical intervention. Atrophic nonunions are a subtype that are particularly difficult to treat. Animal models of atrophic nonunion are available; however, these require surgical or radiation-induced trauma to disrupt periosteal healing. These methods are invasive and not representative of many clinical nonunions where osseous regeneration has been arrested by a "failure of biology". We hypothesized that arresting osteoblast cell proliferation after fracture would lead to atrophic nonunion in mice. Using mice that express a thymidine kinase (tk) "suicide gene" driven by the 3.6Col1a1 promoter (Col1-tk), proliferating osteoblast lineage cells can be ablated upon exposure to the nucleoside analog ganciclovir (GCV). Wild-type (WT; control) and Col1-tk littermates were subjected to a full femur fracture and intramedullary fixation at 12 weeks age. We confirmed abundant tk+ cells in fracture callus of Col-tk mice dosed with water or GCV, specifically many osteoblasts, osteocytes, and chondrocytes at the cartilage-bone interface. Histologically, we observed altered callus composition in Col1-tk mice at 2 and 3 weeks postfracture, with significantly less bone and more fibrous tissue. Col1-tk mice, monitored for 12 weeks with in vivo radiographs and micro-computed tomography (μCT) scans, had delayed bone bridging and reduced callus size. After euthanasia, ex vivo μCT and histology showed failed union with residual bone fragments and fibrous tissue in Col1-tk mice. Biomechanical testing showed a failure to recover torsional strength in Col1-tk mice, in contrast to WT. Our data indicates that suppression of proliferating osteoblast-lineage cells for at least 2 weeks after fracture blunts the formation and remodeling of a mineralized callus leading to a functional nonunion. We propose this as a new murine model of atrophic nonunion. © 2021 American Society for Bone and Mineral Research (ASBMR).

Diagnosis, treatment and etiology analysis of nerve compression by bone fragment after lumbar spine surgery

To explore the diagnosis, treatment, cause and prevention of nerve compression by bone fragment after lumbar spine surgery. The clinical data of 23 patients with nerve compression by bone fragment after lumbar spine surgery from February 2012 to March 2019 were collected retrospectively, including 9 males and 14 females, aged 42 to 81 years with an average of (62.60±5.70) years. The surgical methods included lumbar interbody fusion in 20 cases and spinal endoscopy in 3 cases. All 23 patients experienced radiating pain on the decompression side or the contralateral limb after operation. The time of occurrence was from immediately after operation to 2 weeks after operation, with an average of (3.2±1.7) days. All patients underwent postoperative examination of lumbar spine CT or MRI to confirm residual ectopic bone fragments, and at the same time, bilateral lower extremity color Doppler ultrasound excluded thrombosis. Sources of ectopic bone fragments:14 cases of residual bone fragments caused by intervertebral fusion bone graft loss or fenestration fusion, 6 cases of fractured upper articular process head, and 3 cases of upper articular process bone remaining during spinal endoscopic surgery. The patient's hospital stay was 10 to 37 (23.4±6.2) days. All patients were followed up for 6 to 25 (13.6±3.4) months. Three patients underwent posterior open nerve root exploration for removing bone fragments on the same day or the second day after surgery, and the symptoms were relieved. Twenty patients underwent conservative treatment firstly, and 13 patients were discharged after pain relieved by conservative treatment, 7 patients failed conservative treatment, the 2 cases of failed 7 cases had undergone nerve root block surgery during conservative treatment. Two patients underwent spinal endoscopy nerve root exploration and bone mass removal, and five patients underwent posterior open nerve root exploration and bone fragmentation removal. All postoperative pain symptoms were relieved. Preoperative CT, MRI and intraoperative bone fragment removal confirmed the shape and location of the bone fragments. The most likely source of bone fragments was the loss of intervertebral fusion bone grafts or residual bone fragments resulting from fenestration fusion (14 cases), fractured upper articular process head (6 cases), and upper articular process bones remaining in endoscopic surgery (3 cases). According to the Macnab criteria in evaluating clinical outcome, 20 cases got excellent results and 3 good. After the lumbar spine surgery, the nerve compression by bone fragments is treated with appropriate treatments, and good clinical results can be obtained. Timely removal of residual bone fragments during operation and careful exploration of nerve roots before closing incision can avoid such complications.

Dynamics of spinal cord compression with different patterns of thoracolumbar burst fractures: Numerical simulations using finite element modelling

BackgroundIn thoracolumbar burst fractures, spinal cord primary injury involves a direct impact and energy transfer from bone fragments to the spinal cord. Unfortunately, imaging studies performed after the injury only depict the residual bone fragments position and pattern of spinal cord compression, with little insight on the dynamics involved during traumas. Knowledge of underlying mechanisms could be helpful in determining the severity of the primary injury, hence the extent of spinal cord damage and associated potential for recovery. Finite element models are often used to study dynamic processes, but have never been used specifically to simulate different severities of thoracolumbar burst fractures. MethodsPreviously developed thoracolumbar spine and spinal cord finite element models were used and further validated, and representative vertebral fragments were modelled. A full factorial design was used to investigate the effects of comminution of the superior fragment, presence of an inferior fragment, fragments rotation and velocity, on maximum Von Mises stress and strain, maximum major strain, and pressure in the spinal cord. FindingsFragment velocity clearly was the most influential factor. Fragments rotation and presence of an inferior fragment increased pressure, but rotation decreased both strains outputs. Although significant for both strains outputs, comminution of the superior fragment isn't estimated to influence outputs. InterpretationThis study is the first, to the authors' knowledge, to examine a detailed spinal cord model impacted in situ by fragments from burst fractures. This numeric model could be used in the future to comprehensively link traumatic events or imaging study characteristics to known spinal cord injuries severity and potential for recovery.

Residual bone fragments in tibiofibular joint and postoperative local recurrence: an analysis of 21 cases of proximal fibular giant cell tumour

BackgroundCurrently, there are no known reports on the aetiology of local giant cell tumour (GCT) recurrence in the proximal fibula following en bloc resection. We analysed 21 cases of proximal fibular GCT, focusing on the presence of residual bone in the tibiofibular joint, its causes and its impact on postoperative recurrence.MethodsWe retrospectively analysed 21 cases with proximal fibular GCT occurring between 2000 and 2017.ResultsThere were 14 males and 7 females. The average patient age was 25.0 years. Seventeen patients were diagnosed and treated at our facility, while 4 were referred after local recurrence.Six patients presented with residual bone fragments in the tibiofibular joint during their first month of follow-up. Patients with residual bone fragments had a higher local recurrence rate (83.3%) than those without (0%, p = 0.0003). Upon further analysis, patients with a preoperative Campanacci grade III tumour (p = 0.0055) and pathological fractures (p = 0.0109) were at a higher risk of exhibiting postoperative residual bone fragments.ConclusionsThe presence of residual bone fragments in the tibiofibular joint was the main cause of postoperative local recurrence. The presence of residual bone fragments may be related to the preoperative Campanacci grade and pathological fractures. Therefore, close attention should be paid to postoperative follow-up examinations, and if recurrence is suspected, surgical resection should be planned.

Efficacy of modified 270°spinal canal decompression combined with one-stage pedicle screw fixation and vertebral reconstruction for severe thoracolumbar fractures

Objective To investigate the efficacy of modified 270°spinal canal decompression combined with one-stage pedicle screw fixation and vertebral reconstruction for severe thoracolumbar fractures. Methods A retrospective case-control study was made on 44 cases of severe thoracolumbar fractures treated from January 2009 to June 2014. There were 34 males and 10 females, with age range of 20-68 years (mean, 37.9 years). The injured vertebrae included T11/12 in one case, T12-L1 in 10, L1/2 in 21, L2/3 in nine and L3/4 in three. Injury type was all AO type B2. According to the American Spinal Injury Association (ASIA), neurological deficit was Grade A in three cases, Grade B in five, Grade C in 22, and Grade D in 14. Load sharing score (LSC) was 7-9 points (mean, 8.1 points), while the thoracolumbar injury classification and severity score (TLICS) was 7-9 points (mean, 8.2 points). According to the difference of surgical procedures, all cases were divided into traditional group (21 cases) and improved group (23 cases). The patients in traditional group were treated by 270° spinal canal decompression combined with one-stage pedicle screw fixation and vertebral reconstruction, and those in improved group were treated by modified 270°spinal canal decompression combined with one-stage pedicle screw fixation and vertebral reconstruction. Between-group differences were compared with regard to operation time, intraoperative blood loss, postoperative drainage, allogeneic blood transfusion, pain visual analogue scale (VAS) at postoperative 72 hours, Cobb angle, anterior vertebral height, spinal canal compromise, Denis score, work state, neurological function, bone graft fusion and complications. Results Duration of follow-up was (32.4±15.8)months (range, 12-60 months). No wound infection or deterioration of neurological function was found after operation. Operation time was (2.4±0.5)hours in traditional group, less than (2.8±0.6)hours in improved group (P 0.05). VAS was (3.2±0.9)points in traditional group and (3.3±0.9)points in improved group at postoperative 72 hours (P>0.05). Cobb angle, anterior vertebral height and spinal canal compromise in both groups obtained well recovery and maintained after operation (P 0.05). At the last follow-up, ASIA Grade A was noted in three cases, Grade C in two, Grade D in 23, and Grade E in 16. Bone union was achieved in all cases, with no implant loosening or breakage observed. Thirteen cases in traditional group were found with residual bone fragments in the spinal canal after surgery, and two of them received revision surgery. Conclusions Either modified or traditional 270° spinal canal decompression combined with one-stage pedicle screw fixation and vertebral reconstruction can attain satisfactory clinical results in treatment of severe thoracolumbar fractures. However, the modified 270°spinal canal decompression can achieve more thorough decompression without reducing stability of the spine. Key words: Spinal fractures; Thoracic vertebrae; Lumbar vertebrae; Fracture fixation, internal

Differentiation of spinal giant cell tumors from chordomas by using a scoring system.

Few reports have compared the clinical features and imaging characteristics of giant cell tumor and chordoma of the spine. The aim of the present study was to investigate whether the two types of tumors could be differentially diagnosed, by comparing clinical characteristics as well as magnetic resonance imaging (MRI) or computed tomography (CT) findings and then scoring the characteristic findings. A total of 18 patients were retrospectively assessed. To elucidate the characteristic findings, we investigated the following 10 items: age at diagnosis, sex, and site of occurrence; for MRI findings, the pattern of tumor expansion, T1-weighted images, T2-weighted images, septal structure, and cystic changes; and for CT findings, calcification or residual bone fragments and incomplete bone shells. Then, we developed a unique scoring system and investigated whether the two tumors could be differentiated by this scoring system. Six items, including, age, site of occurrence, tumor expansion pattern, T2-weighted images, septal structure, and incomplete bone shells, were significantly different between giant cell tumor and chordoma patients. By using newly developed scoring system, the mean scores of 0.9±0.6 (range 0-2) for giant cell tumor and 4.8±1.5 (range 3-6) for chordoma patients were significantly different (P<0.001), thereby allowing the differential diagnosis by setting the cutoff value to three. We found that the six items were useful for differentially diagnosing giant cell tumor and chordoma. These results indicate that it may be possible to distinguish the two types of tumor by scoring these items.

The endoscopic endonasal approach to the odontoid and its impact on early extubation and feeding.

The gold-standard surgical approach to the odontoid is via the transoral route. This approach necessitates opening of the oropharynx and is associated with risks of infection, and swallowing and breathing complications. The endoscopic endonasal approach has the potential to reduce these complications as the oral cavity is avoided. There are fewer than 25 such cases reported to date. The authors present a consecutive, single-institution series of 9 patients who underwent the endonasal endoscopic approach to the odontoid. The charts of 9 patients who underwent endonasal endoscopic surgery to the odontoid between January 2005 and August 2013 were reviewed. The clinical presentation, radiographic findings, surgical management, complications, and outcome, particularly with respect to time to extubation and feeding, were analyzed. Radiographic measurements of the distance between the back of the odontoid and the front of the cervicomedullary junction (CMJ) were calculated, as well as the location of any residual bone fragments. There were 7 adult and 2 pediatric patients in this series. The mean age of the adults was 54.8 years; the pediatric patients were 7 and 14 years. There were 5 females and 4 males. The mean follow-up was 42.9 months. Symptoms were resolved or improved in all but 1 patient, who had concurrent polyneuropathy. The distance between the odontoid and CMJ increased by 2.34 ± 0.43 mm (p = 0.03). A small, clinically insignificant fragment remained after surgery, always on the left side, in 57% of patients. Mean times to extubation and oral feeding were on postoperative Days 0.3 and 1, respectively. There was one posterior cervical wound infection; there were 2 cases of epistaxis requiring repacking of the nose and no instances of breathing or swallowing complications or velopharyngeal insufficiency. This series of 9 cases of endonasal endoscopic odontoidectomy highlights the advantages of the approach in permitting early extubation and early feeding and minimizing complications compared with transoral surgery. Special attention must be given to bone on the left side of the odontoid if the surgeon is standing on the right side.

Use of isocentric fluoroscopy during transoral odontoidectomy

The intra-operative isocentric C-arm (Siremobil Iso-C 3D; Siemens Medical Solutions, Erlangen, Germany) allows for the acquisition and immediate interpretation of multiplanar three-dimensional images. The utility of isocentric fluoroscopy in surgery of the spine has been well described, in patients with percutaneous kyphoplasty, anterior cervical discectomy and fusion, fracture repair, as well as in percutaneous instrumentation at cervical, thoracic and lumbar levels. Two patients who underwent transoral odontoidectomy are presented in whom isocentric fluoroscopy was utilized to provide an intra-operative CT scan. Use of this technology allowed the surgeon to detect residual bone peri-operatively, thus allowing for a more extensive decompression and excellent clinical results. Intra-operative isocentric fluoroscopy in transoral odontoidectomy allows for peri-operative detection of residual bone fragments and intra-operative confirmation of complete decompression.

Dual energy CT of peripheral arteries: Effect of automatic bone and plaque removal on image quality and grading of stenoses

Purpose To evaluate the effect of automatic bone and plaque removal on image quality and grading of steno-occlusive lesions in patients undergoing dual energy CT angiography (CTA) of lower extremity. Materials and methods Dual energy (DE) runoff CTA was performed in 50 patients using the following parameters: collimation 2 × 32 × 0.6; tube potentials, 80 kV and 140 kV; reconstructed slice thickness 1 mm. 100 mL iomeprol 400 and 50 mL saline were injected at 4 mL/s. Separate datasets were calculated for each of the two tubes and used to generate automatically bone-subtracted images (ABS) as well as bone and plaque subtracted images (ABPS). Residual bone in the ABS dataset was removed manually (=ABS-B dataset). In addition, a weighted average dataset from both dual energy acquisitions resembling a routine 120 kV CT acquisition was used for standard manual bone subtraction (MBS). Operator time for bone removal was measured. Effectiveness of bone subtraction and presence of vessel erosions was assessed by two readers in consensus. Stenosis grading in plaque subtracted and unsubtracted images was assessed and correlated. Results Residual bone fragments (ribs: 46%, patella: 25%, spine: 4%, pelvis: 2%, tibia 2% of patients) were only observed with ABS. The time needed to manually remove these residual bones was 2.1 ± 1.1 min and was significantly lower than the duration of manual bone removal (6.8 ± 2.0 min, p < 0.0001, paired t-test). A total of 1159 arteries were analyzed. Compromising vessel erosions were observed less frequently in the ABS-B dataset (10.6%) than in the MBS dataset (15.2%, p < 0.001, wilcoxon’s signed rank test). A total of 817 steno-occlusive lesions were assessed. While the agreement of grading of steno-occlusive lesions was good at the levels of the aorta and the pelvic arteries ( κ = 0.70 in both, Cohen’s kappa statistics), it was moderate at the level of the thigh arteries ( κ = 0.57) and poor at the level of the calf ( κ = 0.16). Conclusion DE CTA has substantial advantages over conventional CTA. Automatic bone subtraction is more time efficient and reliable. Automatic plaque subtraction for the first time provides a true CTA-luminogram which is easy to interpret and reduces the need for further post-processing. DE CTA provides best results in arteries of the thigh; below the knee, plaque subtraction is less accurate.

Override of the Osteoclast Defect in Osteopontin-Deficient Mice by Metastatic Tumor Growth in the Bone

Osteopontin (OPN) is a major noncollagenous protein of bone that is frequently up-regulated in tumors, where it enhances tumor growth. OPN-deficient mice are resistant to stimulated bone resorption, including that occurring after ovariectomy. Using a new syngeneic model of bone metastasis (r3T), we examined whether OPN-deficient mice are similarly resistant to bone loss resulting from osteolytic tumor growth. Transformed mammary epithelial cells, r3T, which express parathyroid hormone-related protein but not receptor activator of nuclear factor-kappaB ligand, were injected via the intracardiac route into both wild-type and OPN-/- mice. We measured tumor burden in the bone by quantitative polymerase chain reaction assay and evaluated bone loss by X-ray and microCT. Unexpectedly, bone loss was similar in OPN-/- and wild-type mice bearing similar-sized tumors. Osteoclast number was comparable in both genotypes, and the expression of bone sialoprotein was similar in tumor-bearing bones of both genotypes, excluding two potential mechanisms of overriding the defect. Taken together, these results indicate that in the absence of OPN, the bone loss associated with tumor growth at the bone site proceeds rapidly despite the osteoclast defects documented in OPN-/- mice, suggesting that the mechanism of bone loss due to tumor growth differs from that occurring in other pathologies.

Slurry Method for Rapid DCP Atomic Emission Determination of Residual Bone Fragments in Mechanically Separated Turkey

A slurry atomization direct current plasma (DCP) emission method is reported which involves a minimum of sample preparation for the spectroscopic determination of residual bone solids (as calcium) in mechanically separated turkey. Lengthy wet-ashing methods have been replaced here by a simple approach involving a 2-min “Polytron” homogenization step followed by shaking of the meat homogenate for ∼10 s with dilute nitric acid at room temperature. The sample bulk remains as a suspension of solids that is immediately and directly pumped to a clog-free Babington-type nebulizer installed in a three-electrode DCP emission spectrometer for Ca determination. Before homogenizing, mechanically separated turkey (MST) normally contains bone fragments in the particle size range 0.1 to 0.8 mm. For these MST samples, quantitative Ca recoveries are achieved by the new matrix-modified slurry atomization DCP atomic emission method within about 10 min of the time the meat product is received for bone solids analysis by the lab.

The localization of 1,25-dihydroxycholecalciferol in bone cell nuclei of rachitic chicks

1. A simple technique has been developed to obtain subcellular fractions of chick bone. The method yielded 60-70% of total DNA in the nuclear debris fraction and 80-90% of total (14)C recovered in bone after a dose of radioactive vitamin D. 2. After a dose of [4-(14)C,1,2-(3)H(2)]cholecalciferol (0.5mug) was given to vitamin D-deficient chicks, the time-course of total (14)C radioactivity in the epiphysis, metaphysis and diaphysis of proximal tibiae was measured. The maximum concentrations were reached at 6h, corresponding to a similar peak of radioactivity in blood, decreasing until 24h and indicating the dependence on the circulating (14)C and on the blood supply of the three bone components. 3. The (14)C radioactivity of cholecalciferol and 25-hydroxycholecalciferol (expressed per mg of DNA) followed the pattern of incorporation of total (14)C radioactivity in all three bone components. The more polar metabolite fraction reached a peak of radioactivity at 6-9h and maintained its concentration over the 24h period studied in all anatomical bone components. 4. After a dose of [4-(14)C,1-(3)H]cholecalciferol (0.5mug) was given to vitamin D-deficient chicks, the subcellular distribution was studied. At 24h after dosing, the nuclear fraction contained 27% and the supernatant fraction had 67% of total (14)C recovered in the bone filtrate. When the (14)C in the residual bone fragments was included, the nuclear fraction contained up to 35% of the total radioactivity in the bone. 5. The subcellular distribution pattern of individual vitamin D metabolites indicated that the purified nuclear fraction concentrated the polar metabolite, which lost (3)H at C-1, so that 77% of the radioactivity could be accounted for by 1,25-dihydroxycholecalciferol. The supernatant fraction contained smaller amounts of 1,25-dihydroxycholecalciferol (9%), with 66% of 25-hydroxycholecalciferol forming the major metabolite, corresponding to its concentration found in blood at 24h. 6. The preferential accumulation of 1,25-dihydroxycholecalciferol in the nuclear fraction and the overall pattern of other metabolites, found previously in intestinal tissue, suggests a similar mechanism of action in bone to that postulated for the intestinal cell in calcium translocation.