Evaluation of a method to quantify posture and scapula position using biplanar radiography.

BACKGROUND: Recent studies have stated the relevance of having new parameters to quantify the position and orientation of the scapula with patients standing upright. Although biplanar radiography can provide 3D reconstructions of the scapula and the spine, it is not yet possible to acquire these images with patients in the same position. METHODS: Two pairs of images were acquired, one for the 3D reconstruction of the spine and ribcage and one for the 3D reconstruction of the scapula. Following 3D reconstructions, scapular alignment was performed in two stages, a coarse alignment based on manual annotations of landmarks on the clavicle and pelvis, and an adjusted alignment. Clinical parameters were computed: protraction, internal rotation, tilt and upward rotation. Reproducibility was assessed on an in vivo dataset of upright biplanar radiographs. Accuracy was assessed using supine cadaveric CT-scans and digitally reconstructed radiographs. FINDINGS: The mean error was less than 2° for all clinical parameters, and the 95 % confidence interval for reproducibility ranged from 2.5° to 5.3°. INTERPRETATION: The confidence intervals were lower than the variability measured between participants for the clinical parameters assessed, which indicates that this method has the potential to detect different patterns in pathological populations.

3D reconstruction of the scapula from biplanar X-rays for pose estimation and morphological analysis.

BACKGROUND: Patient-specific scapular shape in functional posture can be highly relevant to clinical research. Biplanar radiography is a relevant modality for that purpose with already two existing assessment methods. However, they are either time-consuming or lack accuracy. The aim of this study was to propose a new, more user-friendly and accurate method to determine scapular shape. METHODS: The proposed method relied on simplified manual inputs and an upgraded version of the first 3D estimate based on statistical inferences and Moving-Least Square (MLS) deformation of a template. Then, manual adjustments, with real-time MLS algorithm and contour matching adjustments with an adapted minimal path method, were added to improve the match between the projected 3D model and the radiographic contours. The accuracy and reproducibility of the method were assessed (with 6 and 12 subjects, respectively). FINDINGS: The shape accuracy was in average under 2 mm (1.3 mm in the glenoid region). The reproducibility study on the clinical parameters found intra-observer 95% confidence intervals under 3 mm or 3° for all parameters, except for glenoid inclination and Critical Shoulder Angle, ranging between 3° and 6°. INTERPRETATION: This method is a first step towards an accurate reconstruction of the scapula to assess clinical parameters in a functional posture. This can already be used in clinical research on non-pathologic bones to investigate the scapulothoracic joint in functional position.

Spontaneous induced bone fusion in minimally invasive fusionless bipolar fixation in neuromuscular scoliosis: a computed tomography analysis.

PURPOSE: Posterior spinal fusion (PSF) at skeletal maturity is still the gold standard in children with neuromuscular scoliosis (NMS) who underwent fusionless surgery. The aim of this computed tomography (CT) study was to quantify the spontaneous bone fusion at the end of a lengthening program by minimally invasive fusionless bipolar fixation (MIFBF), that could avoid PSF. METHODS: NMS operated on with MIFBF from T1 to the pelvis and at final lengthening program were included. CT was performed at least five years postoperatively. The autofusion was classified as completely or not fused at the facets joint (on both coronal and sagittal plane, right and left side, from T1 to L5), and around the rods (axial plane, right and left side, from T5 to L5). Vertebral body heights were assessed. RESULTS: Ten patients were included (10.7y ± 2 at initial surgery). Mean Cobb angle was 82 ± 20 preoperatively and 37 ± 13 at last follow-up. CT were performed on average 6.7y ± 1.7 after initial surgery. Mean preoperative and last follow-up thoracic vertebrae height were respectively 13.5 mm ± 1.7 and 17.4 mm ± 1.7 (p < 0.001). 93% facets joints were fused (out of 320 analyzed joints), corresponding to 15/16 vertebral levels. Ossification around the rods was observed in 6.5±2.4 levels out of 13 in the convex side, and 4.2 ± 2.2 in the concave side (p = 0.04). CONCLUSIONS: This first computed quantitative study showed MIFBF in NMS preserved spinal growth, while it induced 93% of facet joints fusion. This could be is an additional argument when questionning the real need for PSF at skeletal maturity.

3D quasi-automatic spine length assessment using low dose biplanar radiography after surgical correction in thoracic idiopathic scoliosis.

OBJECTIVE: Surgical correction of thoracic scoliosis leads to a height improvement. Our objectives were to assess how the linear and developed spinal column lengths relate to the frontal and sagittal parameters after a surgical correction of thoracic idiopathic scoliosis, and whether the measurement of these lengths is reliable using quasi-automatic 3D reconstruction methods with biplanar X-rays. METHODS: Consecutive children with thoracic idiopathic scoliosis who underwent spinal fusion surgery and biplanar pre and postoperative X-rays in free-standing position were included prospectively. Quasi-automatic computed 3D reconstructions of the spine were done using a previously validated technique and allowed the automatic computation of geometrical spinopelvic parameters including OD-pelvis, linear, and developed T1-T12 and T1-L5 lengths. RESULTS: Thirty patients with scoliosis were included, and 240 reconstructions were performed (2 operators x2 repetitions x30 patients pre and postoperative). The main thoracic Cobb angle, T1-T12, T1-L5 linear and developed distance, OD-pelvis were significantly improved (p < 0.001). The gain of the main thoracic Cobb angle (31.6°;SD = 9°) was correlated to the gain of the linear distance T1-T12 (15.3 mm;SD=7.3 mm)(rho = 0.76;p < 0.0001) and T1-L5 (24.7 mm;SD = 8 mm)(rho = 0.64;p < 0.0001). The postoperative change of developed length between T1-L5 represented 41% of the gain in linear distance between the same vertebrae. Similarly, the gain of T1-T12 developed length was 50% of linear T1-T12 height gain. Both differences were significant (p = 0.01). Absolute bias using Bland & Altman plots was lower than 1 mm for linear distance (0.1%) and lower than 2 mm (0.3%) for developed distance. CONCLUSION: The gain in spinal length is correlated to the thoracic Cobb angle correction in the surgical treatment of idiopathic thoracic scoliosis. The new significant finding is that the developed spinal height gain represented approximately a little less than 50% of the linear spinal height gain and these parameters were reliable from a 3D quasi-automatic reconstruction of biplanar X-ray.

Biomechanical specimen assessment by low dose biplanar X-ray study of fusion constructions using a posterior lumbar cage with integrated anchors and posterior adjunctive fixators.

The objective was to compare L4/5 range of motions of fusion constructs using anchored cages. Twelve human cadaveric spine were tested in intact condition, and divided into TLIF and PLIF groups. Testing consisted in applying pure moments in flexion-extension, lateral bending and axial rotation. The computation of intersegmental motion was assessed using 3 D biplanar radiographs. In TLIF group, the addition of contralateral transfacet decreased flexion-extension motion (39%; p = 0.036) but without difference with the ipsilateral pedicle screw construction (53%; p = 0.2). In PLIF group, the addition of interspinous anchor reduced flexion-extension motion (12%; p = 0.036) but without difference with the bilateral pedicle screw construction (17%; p = 0.8).

Effect of postural alignment alteration with age on vertebral strength.

EOS biplane radiographs of 117 subjects between 20 and 83 years were analyzed to compute the upper body lever arm over the L1 vertebra and its impact on vertebral strength. Postural sagittal alignment alteration was observed with age and resulted in a greater lever arm causing vertebral strength to decrease. PURPOSE: The purpose of this study was to analyze the impact of postural alignment changes with age on vertebral strength using finite element analysis and barycentremetry. METHODS: A total of 117 subjects from 20 to 83 years were divided in three age groups: young (20 to 40 years, 62 subjects), intermediate (40 to 60 years, 26 subjects), and elderly (60 years and over, 29 subjects). EOS biplane radiographs were acquired, allowing 3D reconstruction of the spine and body envelope as well as spinal, pelvic, and sagittal alignment parameter measurements. A barycentremetry method allowed the estimation of the mass and center of mass (CoM) position of the upper body above L1, relatively to the center of the L1 vertebra (lever arm). To investigate the effect of this lever arm, vertebral strength of a generic finite element model (with constant geometry and mechanical properties for all subjects) was successively computed applying the personalized lever arm of each subject. RESULTS: A combination of an increase in thoracic kyphosis, cervical lordosis, and pelvic tilt with a loss of lumbar lordosis was observed between the young and the older groups. Sagittal alignment parameters indicated a more forward position as age increased. The lever arm of the CoM above L1 varied from an average of 1 mm backward for the young group, to averages of 10 and 24 mm forward, respectively, for the intermediate and elderly group. As a result, vertebral strength decreased from 2527 N for the young group to 1820 N for the elderly group. CONCLUSION: The global sagittal alignment modifications observed with age were consistent with the literature. Posture alteration with age reduced vertebral strength significantly in this simplified loading model. Postural alignment seems essential to be considered in the evaluation of osteoporotic patients.

Analysis of apex and transitional vertebra of the spine according to pelvic incidence using orientation and position parameters.

OBJECTIVE: To identify the different apex and transitional vertebra according to the shape of the pelvis of individuals despite their difference in sagittal alignment using our measurement system. METHODS: Full-spine X-rays using EOS in standard stand-position of 99 volunteers were selected (47 women, 52 men, mean age 31 years old). Validated 3D reconstruction technique allows extraction of spinopelvic parameters, and position and rotation of each vertebra and lumbar disks. Subjects were divided into three groups: low PI (lowPI, n = 37), moderate PI (midPI, n = 52), high PI (highPI, n = 10), with, respectively, a PI below 45°, between 45° and 60° and above 60°. Occurrence of specific position and rotation values of apex and transitional vertebra were assessed in each group. RESULTS: Frequency curves tend to move cranially when the incidence increases except in cervicothoracic where T1 is a constant for all shapes of spine with occurrence approaching 90%. Angulation value of relevant vertebra and lumbar lordosis are significantly positively correlated for the whole population. CONCLUSIONS: Our study allowed the assessment of the distribution of spine curvatures according to the pelvic incidence. It describes the occurrence of localization of the apex and transitional vertebrae according to pelvic incidence. These results should be taken into account during the analysis of the sagittal balance, especially when planning deformity surgery in adults.

Biomechanical cadaver study of proximal fixation in a minimally invasive bipolar construct.

STUDY DESIGN: Biomechanical human cadaver study. OBJECTIVE: To determine the three-dimensional intervertebral ranges of motion (ROMs) of intact and hook-instrumented thoracic spine specimens subjected to physiological loads, using an in vitro experimental protocol with EOS biplane radiography. Pedicle screws are commonly used in thoracic instrumentation constructs, and their biomechanical properties have been widely studied. Promising clinical results have been reported using a T1-T5 thoracic hook-claw construct for proximal rod anchoring. Instrumentation stability is a crucial factor in minimizing mechanical complications rates but had not been assessed for this construct in a biomechanical study. METHODS: Six fresh-frozen human cadaver C6-T7 thoracic spines were studied. The first thoracic vertebrae were instrumented using two claws of supra-laminar and pedicle hooks, each fixed on two adjacent vertebrae, on either side of a single free vertebra. Quasi-static pure-moment loads up to 5 Nm were applied to each specimen before and after instrumentation, in flexion-extension, right and left bending, and axial rotation. Five steel beads impacted in each vertebra allowed 3D tracking of vertebral movements on EOS biplanar radiographs acquired after each loading step. The relative ranges of motion (ROMs) of each pair of vertebras were computed. RESULTS: Mean ROMs with the intact specimens were 17° in flexion-extension, 27.9° in lateral bending, and 29.5° in axial rotation. Corresponding values with the instrumented specimens were 0.9°, 2.6°, and 7.3°, respectively. Instrumentation significantly (P < 0.05) decreased flexion-extension (by 92-98%), lateral bending (by 87-96%), and axial rotation (by 68-84%). CONCLUSION: This study establishes the biomechanical stability of a double claw-hook construct in the upper thoracic spine, which may well explain the low mechanical complication rate in previous clinical studies. LEVEL OF EVIDENCE: Not applicable, experimental cadaver study.

Experimental analysis of the lower cervical spine in flexion with a focus on facet tracking.

Cervical traumas are among the most common events leading to serious spinal cord injuries. While models are often used to better understand injury mechanisms, experimental data for their validation remain sparse, particularly regarding articular facets. The aim of this study was to assess the behavior of cervical FSUs under quasi-static flexion with a specific focus on facet tracking. 9 cadaveric cervical FSUs were imaged and loaded under a 10 Nm flexion moment, exerted incrementally, while biplanar X-rays were acquired at each load increment. The relative vertebral and facet rotations and displacements were assessed using radio-opaque markers implanted in each vertebra and CT-based reconstructions registered on the radiographs. The only failures obtained were due to specimen preparation, indicating a failure moment of cervical FSUs greater than 10 Nm in quasistatic flexion. Facet motions displayed a consistent anterior sliding and a variable pattern regarding their normal displacement. The present study offers insight on the behavior of cervical FSUs under quasi-static flexion beyond physiological thresholds with accurate facet tracking. The data provided should prove useful to further understand injury mechanisms and validate models.

Biplanar stereoradiography predicts pulmonary function tests in adolescent idiopathic scoliosis: a cross-sectional study.

PURPOSE: Various spinal and rib cage parameters measured from complex examinations were found to be correlated with preoperative pulmonary function tests (PFT). The aim was to investigate the relationship between preoperative rib cage parameters and PFT using biplanar stereoradiography in patients with severe adolescent idiopathic scoliosis. METHODS: Fifty-four patients, 45 girls and nine boys, aged 13.8 ± 1.2 years, with Lenke 1 or 2 thoracic scoliosis (> 50°) requiring surgical correction were prospectively included. All patients underwent preoperative PFT and low-dose biplanar X-rays. The following data were collected: forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), FEV1/FVC ratio, residual volume, slow vital capacity (SVC), total lung capacity (TLC), rib cage volume (RCV), maximum rib hump, maximum width, mean thoracic index, spinal penetration index, apical vertebral rotation, main curve Cobb angle (MCCA), T4-T12 kyphosis. The primary outcome was the relationship between rib cage parameters and PFT. The secondary outcome was the relationship between rib cage parameters and spine parameters. Data were analyzed using Spearman's rank test. A multivariable regression analysis was performed to compare PFTs and structural parameters. Significance was set at α = 0.05. RESULTS: The mean MCCA was 68.7° ± 16.7°. RCV was highly correlated with all pulmonary capacities: TLC (r = 0.76, p < 0.0001), SVC (r = 0.78, p < 0.0001) and FVC (r = 0.77, p < 0.0001). RCV had a low correlation with FEV1/FVC (r = - 0.34, p = 0.014). SPI was not correlated with any pulmonary parameters. CONCLUSION: Rib cage volume measured by biplanar stereoradiography may represent a prediction tool for PFTs. LEVEL OF EVIDENCE: Non-randomized cross-sectional study among consecutive patients, Level 2. These slides can be retrieved under Electronic Supplementary Material.

Validity and reliability of different techniques of neck-shaft angle measurement.

AIM: To determine a valid and reliable neck-shaft angle (NSA) measurement method while rotating the pelvises in increments of 5° in order to simulate patient malpositioning. MATERIALS AND METHODS: CT images of 17 patients were used to produce digitally reconstructed radiographs in frontal and lateral views and three-dimensional (3D)-reconstructions of the femurs, considered to be the reference standard. Malpositioning was simulated by axially rotating the frontal radiographs from 0° to 20°. Three operators measured in two-dimensions the NSA using four different methods, three times each, at each axial rotation (AR) position. Method 1 (femoral neck axis drawn by joining the centre of the femoral head (CFH) to the median of the femoral neck base; femoral diaphysis axis drawn by joining the median of two lines passing through the medial and lateral edges of the femoral axis below the lesser trochanter) and method 2 (femoral axis taken as the median of a triangle passing through base of femoral neck and medial and lateral head-neck junction; femoral diaphysis as previous) were described for the first time; method 3 was based on a previous study; method 4 was a free-hand technique. Reliability, validity, and global uncertainty were assessed. RESULTS: Method 1 showed the best reliability and validity. The global uncertainty also showed minimal values for method 1, ranging from 7.4° to 14.3° across AR positions. CONCLUSION: Method 1, based on locating the CFH, was the most reliable and valid method and should be considered as a standardised two-dimensional NSA measurement method for clinical application.

Lower-limb lengths and angles in children older than six years: Reliability and reference values by EOS® stereoradiography.

BACKGROUND: Lower-limb alignment in children is classically assessed clinically or based on conventional radiography, which is associated with projection bias. Low-dose biplanar radiography was described recently as an alternative to conventional imaging. The primary objective of this study was to assess the reliability of length and angle values inferred from 3D reconstructions in children seen in everyday practice. The secondary objective was to obtain reference values for goniometry parameters in children. HYPOTHESIS: 3D reconstructions can be used to assess the lower limbs in children. MATERIAL AND METHODS: The paediatric reliability study was done in 18 volunteers who were divided into three groups based on whether they were typically developing (TD) children, had skeletal development abnormalities, or had cerebral palsy. The reference data were obtained in 129 TD children. Each study participant underwent biplanar radiography with 3D reconstruction performed by experts and radiology technicians. Goniometry parameters were computed automatically. Reproducibility was assessed based on the intra-class coefficient (ICC) and the ISO 5725 standard (standard deviation of reproducibility, SDR). RESULTS: For length parameters, the ICCs ranged from 0.94 to 1.00 and the SDR from 2.1 to 3.5mm. For angle parameters, the ICC and SDR ranges were 0.60-0.95 and 0.9°-4.6°, respectively. No significant differences were found across experts or radiology technicians. Age-specific reference data are reported. DISCUSSION: These findings confirm the reliability of low-dose biplanar radiography for assessing lower-limb parameters in children seen in clinical practice. In addition, the study provides reference data for commonly measured parameters. LEVEL OF EVIDENCE: IV.

Estimation of spinal joint centers from external back profile and anatomical landmarks.

Defining a subject-specific model of the human body is required for motion analysis in many fields, such as in ergonomics and clinical applications. However, locating internal joint centers from external characteristics of the body still remains a challenging issue, in particular for the spine. Current methods mostly require a set of rarely accessible (3D back or trunk surface) or operator dependent inputs (large number of palpated landmarks and landmarks-based anthropometrics). Therefore, there is a need to provide an alternative way to estimate joint centers only using a limited number of easily palpable landmarks and the external back profile. Two methods were proposed to predict the spinal joint centers: one using only 6 anatomical landmarks (ALs) (2 PSIS, T8, C7, IJ and PX) and one using both 6 ALs and the external back profile. Regressions were established using the X-ray based 3D reconstructions of 80 subjects and evaluated on 13 additional subjects of variable anthropometry. The predicted location of joint centers showed an average error 9.7 mm (±5.0) in the sagittal plane for all joints when using the external back profile. Similar results were obtained without using the external back profile, 9.5 mm (±5.0). Compared to other existing methods, the proposed methods offered a more accurate prediction with a smaller number of palpated points. Additional methods have to be developed for considering postures other than standing, such as a sitting position.

In Vivo Assessment of Elasticity of Child Rib Cortical Bone Using Quantitative Computed Tomography.

Elasticity of the child rib cortical bone is poorly known due to the difficulties in obtaining specimens to perform conventional tests. It was shown on the femoral cortical bone that elasticity is strongly correlated with density for both children and adults through a unique relationship. Thus, it is assumed that the relationships between the elasticity and density of adult rib cortical bones could be expanded to include that of children. This study estimated in vivo the elasticity of the child rib cortical bone using quantitative computed tomography (QCT). Twenty-eight children (from 1 to 18 y.o.) were considered. Calibrated QCT images were prescribed for various thoracic pathologies. The Hounsfield units were converted to bone mineral density (BMD). A relationship between the BMD and the elasticity of the rib cortical bone was applied to estimate the elasticity of children's ribs in vivo. The estimated elasticity increases with growth (7.1 ± 2.5 GPa at 1 y.o. up to 11.6 ± 1.9 GPa at 18 y.o.). This data is in agreement with the few previous values obtained using direct measurements. This methodology paves the way for in vivo assessment of the elasticity of the child cortical bone based on calibrated QCT images.

Limiting interpedicular screw displacement increases shear forces in screws: A finite element study.

BACKGROUND CONTEXT: Screw loosening has been reported for non-fusion devices. Forces on pedicle screws could be related to kinematic parameters as the interpedicular displacement (ID), which consists of the displacement between superior and inferior screw heads from full extension to full flexion. PURPOSE: To investigate the relationship between ID and screw loosening for different designs of posterior implants using a finite element model. METHODS: An L3-sacrum previously validated spine FE model was used. Three-rod designs were considered in L4-L5 segment: a rigid screw-rod implant, a flexible one and a specific design with a sliding rod providing limited restrain in ID. In order to simulate intermediate configurations, the friction coefficient between the sliding rods and connectors were varied. The sacrum was rigidly fixed. Rotations (flexion-extension, lateral bending and axial rotation) were applied to L3, for each modeled configuration: intact, injured, injured with different implants. Model consistency was checked with existing experimental in vitro data on intact and instrumented segments. Screw loads were computed as well as ID. RESULTS: In flexion-extension, the ID was less than 2mm for rigid (R) and flexible (F) constructs and 5.5mm for intact spine and the sliding implant (S3). Screw's shear forces were 272N, 153N, 43N respectively for R, F and S3 constructs. CONCLUSIONS: Implants that allow ID presented lower screws loads. A compromise between the ability of the implant to withstand compressive forces, which requires longitudinal stiffness, and its ability to allow ID could be important for future implant designs in order to prevent screw loosening.

Influence of patient rotational malpositioning on pelvic parameters assessed on lateral radiographs.

AIM: To estimate the effect of patients' axial rotation (AR) during pelvic radiograph acquisition, on the reliability and validity of sagittal pelvic parameters. MATERIALS AND METHODS: Lateral digitally reconstructed radiographs (LDRRs) were obtained from the pelvic computed tomography (CT) scans of eight children and nine adults. Then, the AR of the pelvis was simulated and the corresponding LDRRs were reconstructed at 5°, 10°, 15°, and 20° of the AR. Pelvic parameters were measured digitally on each radiograph. Intra- and interobserver variability were evaluated at each AR position (three operators repeated the measurements three times each). The bias on each clinical parameter, in each AR position, was calculated relatively to the 0° position. RESULTS: Interobserver variability increased similarly in children and adults with AR. It reached 4.4° for pelvic incidence and 4.7° for the sacral slope at 20° of AR. Biases on radiological parameters increased with AR and exceeded the acceptable threshold of errors when AR reached 10°. A linear regression was established (R2=0.834, p<0.0001) in order to estimate the AR of a patient on a lateral pelvic radiograph based on the measurement of the bifemoral distance normalized to the sagittal pelvic thickness. CONCLUSIONS: AR of patients during radiograph acquisition can be estimated in clinical practice, which would allow physicians to discard any radiographs where the calculated AR exceeded 10°.

Lumbar spinal muscles and spinal canal study by MRI three-dimensional reconstruction in adult lumbar spinal stenosis.

BACKGROUND: Lumbar spinal stenosis is degenerative disc disease most common manifestation. If stenosis degree seems poorly related to symptom severity, lumbar muscles role is recognized. Many studies report imaging methods, to analyze muscle volumes and fat infiltration (FI), but remain limited due to the difficulty to represent entire muscle volume variability. Recently a 3D muscle reconstruction protocol (using the deformation of a parametric specific object method (DPSO) and three-point Dixon images) was reported. It offers the ability to evaluate, muscles volumes and muscle FI. PURPOSE: To describe, in a lumbar spinal stenosis population, muscle volumes, muscle FI and lumbar spinal canal volume with 3D MRI images reconstructions. MATERIALS AND METHODS: Ten adults presenting L4-L5 lumbar stenosis, were included. After specific MRI protocol, three-dimensional, muscle and spinal canal, reconstructions were performed. Muscle (psoas and paraspinal muscles) volumes and fat infiltration (FI), the spinal canal volume, age, and height were correlated one to each other with Spearman correlation factor. An ANOVA was performed to evaluate the intervertebral level influence (P≤0.05). RESULTS: Muscle volumes correlated with height (r=0.68 for psoas). Muscles FI correlated with age (r=0.66 for psoas) and lumbar spinal canal volume (r=0.91). Psoas and paraspinal volumes were maximum at L3-L4 level whereas FI increased from L1-L2 to L5-S1 level. DISCUSSION: These first results illustrate the importance to consider muscles entirely and report correlations between muscles FI, lumbar spinal canal volume and age; and between muscle volumes and patients height. Muscle degeneration seems more related to muscle FI than muscle volume. LEVEL OF EVIDENCE: 3.

A principal component analysis of the relationship between the external body shape and internal skeleton for the upper body.

Recent progress in 3D scanning technologies allows easy access to 3D human body envelope. To create personalized human models with an articulated linkage for realistic re-posturing and motion analyses, an accurate estimation of internal skeleton points, including joint centers, from the external envelope is required. For this research project, 3D reconstructions of both internal skeleton and external envelope from low dose biplanar X-rays of 40 male adults were obtained. Using principal component analysis technique (PCA), a low-dimensional dataset was used to predict internal points of the upper body from the trunk envelope. A least squares method was used to find PC scores that fit the PCA-based model to the envelope of a new subject. To validate the proposed approach, estimated internal points were evaluated using a leave-one-out (LOO) procedure, i.e. successively considering each individual from our dataset as an extra-subject. In addition, different methods were proposed to reduce the variability in data and improve the performance of the PCA-based prediction. The best method was considered as the one providing the smallest errors between estimated and reference internal points with an average error of 8.3mm anterior-posteriorly, 6.7mm laterally and 6.5mm vertically. As the proposed approach relies on few or no bony landmarks, it could be easily applicable and generalizable to surface scans from any devices. Combined with automatic body scanning techniques, this study could potentially constitute a new step towards automatic generation of external/internal subject-specific manikins.