Evaluation of bone strength: correlation between measurements of bone mineral density and drilling force

Modulation of osteogenic differentiation by Escherichia coli-derived recombinant bone morphogenetic protein-2

Recombinant human bone morphogenetic protein-2 (rhBMP-2), a key regulator of osteogenesis, induces the differentiation of mesenchymal cells into cartilage or bone tissues. Early orthopedic and dental studies often used mammalian cell-derived rhBMP-2, especially Chinese hamster ovary (CHO) cells. However, CHO cell-derived rhBMP-2 (C-rhBMP-2) presents disadvantages such as high cost and low production yield. To overcome these problems, Escherichia coli-derived BMP-2 (E-rhBMP-2) was developed; however, the E-rhBMP-2-induced signaling pathways and gene expression profiles during osteogenesis remain unclear. Here, we investigated the E-rhBMP-2-induced osteogenic differentiation pattern in C2C12 cells and elucidated the difference in biological characteristics between E-rhBMP-2 and C-rhBMP-2 via surface plasmon resonance, western blotting, qRT-PCR, RNA-seq, and alkaline phosphatase assays. The binding affinities of E-rhBMP-2 and C-rhBMP-2 towards BMP receptors were similar, both being confirmed at the nanomolecular level. However, the phosphorylation of Smad1/5/9 at 3 h after treatment with E-rhBMP-2 was significantly lower than that on treatment with C-rhBMP-2. The expression profiles of osteogenic marker genes were similar in both the E-rhBMP-2 and C-rhBMP-2 groups, but the gene expression level in the E-rhBMP-2 group was lower than that in the C-rhBMP-2 group at each time point. Taken together, our results suggest that the osteogenic signaling pathways induced by E-rhBMP-2 and C-rhBMP-2 both follow the general Smad-signaling pathway, but the difference in intracellular phosphorylation intensity results in distinguishable transcription profiles on osteogenic marker genes and biological activities of each rhBMP-2. These findings provide an extensive understanding of the biological properties of E-rhBMP-2 and the signaling pathways during osteogenic differentiation.

Synthesis of Human Bone Morphogenetic Protein-2 (hBMP-2) in E. coli Periplasmic Space: Its Characterization and Preclinical Testing

Human BMP-2, a homodimeric protein that belongs to the TGF- β family, is a recognized osteoinductor due to its capacity of inducing bone regeneration and ectopic bone formation. The administration of its recombinant form is an alternative to autologous bone grafting. A variety of E. coli-derived hBMP-2 has been synthesized through refolding of cytoplasmic inclusion bodies. The present work reports the synthesis, purification, and characterization of periplasmic hBMP-2, obtained directly in its correctly folded and authentic form, i.e., without the initial methionine typical of the cytoplasmic product that can induce undesired immunoreactivity. A bacterial expression vector was constructed including the DsbA signal peptide and the cDNA of hBMP-2. The periplasmic fluid was extracted by osmotic shock and analyzed via SDS-PAGE, Western blotting, and reversed-phase high-performance liquid chromatography (RP-HPLC). The purification was carried out by heparin affinity chromatography, followed by high-performance size-exclusion chromatography (HPSEC). HPSEC was used for qualitative and quantitative analysis of the final product, which showed >95% purity. The classical in vitro bioassay based on the induction of alkaline phosphatase activity in myoblastic murine C2C12 cells and the in vivo bioassay consisting of treating calvarial critical-size defects in rats confirmed its bioactivity, which matched the analogous literature data for hBMP-2.

Drilling Energy Correlates With Screw Insertion Torque, Screw Compression, and Pullout Strength: A Cadaver Study

INTRODUCTION

To determine whether drilling energy correlates with bone mineral density (BMD), maximum insertion torque (MIT), maximum screw compression, and pullout strength (POS).

METHODS

Ten cadaver tibias were used for testing. Unicortical pilot holes were drilled and the drilling energy measured. Drill site bone quality was determined with microcomputed tomography. Drill holes were randomly assigned to POS or MIT testing using 3.5-mm cortical screws engaging only the near cortex. Pearson correlation coefficients were calculated to determine the relationship between drilling energy, BMD, POS, MIT, and maximum screw compression.

RESULTS

Drilling energy was correlated with BMD (P < 0.001). Compared with BMD, drilling energy had a better correlation with MIT, maximum screw compression, and POS. Maximum screw compression also correlated with MIT (P = 0.012).

CONCLUSIONS

Drilling energy better correlates with MIT, maximum screw compression, and POS compared with BMD in cadaver cortical bone. Dynamically measuring drilling energy may help inform the orthopaedic surgeon as to the quality of the bone before insertion of implants.

Human bone morphogenetic protein-2 (hBMP-2) characterization by physical-chemical, immunological and biological assays

Commercially available preparations of methionyl-human BMP-2 and CHO-derived hBMP-2, which belongs to the transforming growth factor β (TGF-β) superfamily, were used for a complete characterization. This protein is an extremely efficient osteoinductor that plays an important role during bone regeneration and embryonic development. Characterization was carried out via SDS-PAGE and Western blotting, followed by reversed-phase HPLC, size-exclusion HPLC and MALDI-TOF-MS. The classical in vitro bioassay, based on the induction of alkaline phosphatase activity in C2C12 cells, confirmed that hBMP-2 biological activity is mostly related to the dimeric form, being ~ 4-fold higher for the CHO-derived glycosylated form when compared with the E. coli counterpart. The E. coli-derived met-hBMP-2 has shown, by MALDI-TOF-MS, a large presence of the bioactive dimer. A more complex molecular mass (MM) distribution was found for the CHO-derived product, whose exact MM has never been reported because of its variable glycosylation. A method based on RP-HPLC was set up, allowing a quantitative and qualitative hBMP-2 determination even directly on ongoing culture media. Considering that hBMP-2 is highly unstable, presenting moreover an extremely high aggregate value, we believe that these data pave the way to a necessary characterization of this important factor when synthesized by DNA recombinant techniques in different types of hosts.

Application of a Nonlinear Hammerstein-Wiener Estimator in the Development and Control of a Magnetorheological Fluid Haptic Device for Robotic Bone Biopsy

A force generator module (FGM) based on magnetorheological fluid (MRF) was developed to provide force-feedback information for applications in tele-robotic bone biopsy procedures. The FGM is capable of rapidly re-producing a wide range of forces that are common in bone biopsy applications. As a result of the nonlinear nature of MRF, developing robust controllers for these mechanisms can be challenging. In this paper, we present a case study motivated by robotic bone biopsy. We use a non-linear Hammerstein-Wiener (H-W) estimator to address this challenge. The case is presented through three studies. First, an experiment to develop design constraints is presented and describes biopsy force measurements for various animal tissues. Required output forces were found to range between <1 N and <50 N. A second study outlines the design of the FGM and presents the experimental characterization of the hysteretic behavior of the MRF. This data is then used as estimators and validators to develop the nonlinear Hammerstein-Wiener (H-W) model of the MRF. Validation experiments found that the H-W model is capable of predicting the behavior of the MRF device with 95% accuracy and can eliminate hysteresis in a closed-loop control system. The third study demonstrates the FGM used in a 1-DOF haptic controller in a simulated robotic bone-biopsy. The H-W control tracked the input signal while compensating for magnetic hysteresis to achieve optimal performance. In conclusion, the MRF-based device can be used in surgical robotic operations that require a high range of force measurements.

Bone Drilling Breakthrough Detection via Energy-Based Signal

In this paper, a novel energy-based signal, the removal energy density $e_{b}$, is proposed as the detecting signal to determine the breakthrough instances in bone drilling based on the energy approach during the process. The proposed signal is derived from the energy conversion and conservation in drilling process. And the relationship between the signal and the drilling parameter, e.g., drilling speed, feed rate and drill bit radius, is derived. Recursive least square with time forgetting factor is used to estimate $e_{b}$ from the drilling parameters and drilling torque information. Unlike the traditional force profile, this proposed signal profile is similar under different drilling parameters including drilling speed and feed rate, hence reducing the difficulty in setting a threshold for the detection. Experiment on porcine bone is performed. The results show that the proposed signal profile is more consistent than the commonly used force profile, verifying the effectiveness of the proposed method.

INVESTIGATION OF BONE DRILLING FOR SECURE IMPLANT FIXATION IN HUMAN FEMURS: TAGUCHI OPTIMIZATION AND PREDICTIVE FORCE MODELS WITH EXPERIMENTAL VALIDATION

Drilling procedures are important to optimize and ensure the strongest fixation in bone fracture treatment and reconstruction surgery. The mechanistic force models currently available for bovine bones, human spines and human mandibles are not relevant to perform drilling through human femurs. The present study addresses this lack of information and aims to develop the predictive force models for drilling human femurs at different regions and directions. In this study, 10 freshly harvested cadaveric human femurs were included, and a surgical drill bit of 3.2[Formula: see text]mm diameter was used to make 4[Formula: see text]mm deep holes. Different spindle speeds (500, 1000 and 1500[Formula: see text]rpm), feed rates (40, 60 and 80[Formula: see text]mm/min), and apparent density between 0.98 and 1.98[Formula: see text]g/cm3were considered. The optimal parameters [Formula: see text], [Formula: see text], and [Formula: see text] respectively obtained for longitudinal, radial, and circumferential direction could minimize the thrust forces in bone drilling by up to 7.70, 10.50, and 16.20 N, respectively. Validation study demonstrated that the force model developed could predict the thrust force from computed tomography data sets of the patient, only with 5.05%, 6.74%, and 4.91% as a maximum error in longitudinal, radial, and circumferential direction. This important tool can assist to perform complicated surgical operations.

Dual Motor Drill Continuously Measures Drilling Energy to Calculate Bone Density and Screw Pull-out Force in Real Time

INTRODUCTION

Low bone density complicates the surgical management of fractures. Screw stripping in osteoporotic bone leads to decreased fixation strength and weakening of the fixation construct. If low density could be detected during drilling, augmentation may be performed to prevent screw stripping. Furthermore, continuous monitoring of the drill bit depth and bone density can allow detection of the far cortex where density suddenly increases, providing immediate and accurate screw length measurement and reducing the risk of overpenetration or plunge in osteoporotic bone. Therefore, a dual motor drill was created to calculate bone density and pull-out force in real time. The purpose of this study was to determine whether real-time monitoring of drill bit torque and depth could be used to estimate bone density and pull-out force. We hypothesized that the calculated drilling energy could be used to determine density and would correlate with pull-out force.

METHODS

Drilling and screw insertion were performed using a validated composite unicortical bone model. Screws of 5-, 10-, and 20-mm length were placed into blocks of known densities (10, 20, 30, and 40 pounds per cubic foot). During creation of holes by the dual motor drill, drilling energy was recorded and used to calculate density. Calculated bone density was then compared with the known density of the block. The drill bit was exchanged for a screwdriver, and screw insertion energy was recorded in a similar fashion during screw placement. Screws were then subjected to maximal axial pull-out force testing with a material testing device. Recorded drilling energy and screw insertion energy were then correlated with the measured pull-out force.

RESULTS

Calculated bone density correlated very strongly with the known control density, confirming the accuracy of density calculations in real time. Drilling energy and screw insertion energy correlated very strongly with the measured pull-out force by destructive testing confirming ultimate pull-out force could be quantified during drilling or placement of a screw.

DISCUSSION

Our results confirmed that a dual motor drill can accurately and immediately allow determination of bone density and screw pull-out force before placing a screw. This knowledge could allow a surgeon to perform augmentation or alter surgical technique to prevent screw stripping and loss of fixation as well as detect the far cortex and prevent overpenetration in osteoporotic bone.

Simulation and evaluation of a bone sawing procedure for orthognathic surgery based on an experimental force model

Bone sawing is widely used in orthognathic surgery to correct maxillary deformities. Successful execution of bone sawing requires a high level of dexterity and experience. A virtual reality (VR) surgical simulator can provide a safe, cost-effective, and repeatable training method. In this study, we developed a VR training simulator with haptic functions to simulate bone-sawing force, which was generated by the experimental force model. Ten human skulls were obtained in this study for the determination of surgical bone-sawing force. Using a 5-DOF machining center and a micro-reciprocating saw, bone specimens with different bone density were sawed at different feed rates (20, 40, and 60 mm/min) and spindle speeds (9800, 11,200 and 12,600 cycles per minute). The sawing forces were recorded with a piezoelectric dynamometer and a signal acquisition system. Linear correlation analysis of all experimental data indicates that there were significant positive linear correlations between bone-sawing force and bone density and tool feed rate and a moderate negative linear correlation with tool spindle rate. By performing multiple regression analysis, the prediction models for the bone-sawing procedure were determined. By employing Omega.6 as a haptic device, a medical simulator for the Lefort I osteotomy was developed based on an experimental force model. Comparison of the force-time curve acquired through experiments and the curve computed from the simulator indicate that the obtained forces based on the experimental force model and the acquired data had the same trend for the bone-sawing procedure of orthognathic surgery.

A New Method to Intra-Operatively Measure Local Bone Strength in Osteoporotic Bone Using a Modified Surgical Tool

The aim of this study was to test a novel method for intra-operative assessment of osteoporotic bone fracture strength using a surgical tool instrumented with a strain gauge and compare the device with cortical width (CW) measurements in the distal radius. The force needed to puncture the cortex (FNP) was quantified with the device and found to strongly correlate with bone mineral density (BMD) in the diaphysis (adj. R2 = 0.66, p < 0.001). Screw pullout studies were performed and correlation between FNP and screw pullout strength (SPS) was modest (adj. R2 = 0.34 with p < 0.05). CW correlated well with BMD (adj. R2 = 0.7, p < 0.0001) and SPS (adj. R2 = 0.5, p = 0.002) in the diaphysis. This technology may allow objective intra-operative assessment of bone strength to provide surgeons another tool for decision making on fixation strategies appropriate to the area of bone treated.

Relationships between tissue properties and operational parameters of a dental handpiece during simulated cavity preparation

A preliminary study was conducted on the development of an intelligent dental handpiece with functionality to detect subtle changes in mechanical properties of tooth tissue during milling. Such equipment would be able to adopt changes in cutting parameters and make real-time measurements to avoid tooth tissue damage caused by overexertion and overextension of the cutting tool. A modified dental handpiece, instrumented with strain gauges, microphone, displacement sensor, and air pressure sensor, was mounted to a linear movement table and used to mill three to four cavities in >50 bovine teeth. Extracted sound frequency and density were analyzed along with force, air pressure, and displacement for correlations and trends. Experimental results showed a high correlation (coefficient close to 0.7) between the feed force, the rotational frequency, and the averaged gray scale. These results could form the basis of a feedback control system to improve the safety of dental cutting procedures. This article is written in memory of Dr Hongyan Sun, who passed away in 2011 at a young age of 37.

Estimation of tool pose based on force-density correlation during robotic drilling

The application of image-guided systems with or without support by surgical robots relies on the accuracy of the navigation process, including patient-to-image registration. The surgeon must carry out the procedure based on the information provided by the navigation system, usually without being able to verify its correctness beyond visual inspection. Misleading surrogate parameters such as the fiducial registration error are often used to describe the success of the registration process, while a lack of methods describing the effects of navigation errors, such as those caused by tracking or calibration, may prevent the application of image guidance in certain accuracy-critical interventions. During minimally invasive mastoidectomy for cochlear implantation, a direct tunnel is drilled from the outside of the mastoid to a target on the cochlea based on registration using landmarks solely on the surface of the skull. Using this methodology, it is impossible to detect if the drill is advancing in the correct direction and that injury of the facial nerve will be avoided. To overcome this problem, a tool localization method based on drilling process information is proposed. The algorithm estimates the pose of a robot-guided surgical tool during a drilling task based on the correlation of the observed axial drilling force and the heterogeneous bone density in the mastoid extracted from 3-D image data. We present here one possible implementation of this method tested on ten tunnels drilled into three human cadaver specimens where an average tool localization accuracy of 0.29 mm was observed.

Biomechanical Measurements of Surgical Drilling Force and Torque in Human Versus Artificial Femurs

Few experimental studies have examined surgical drilling in human bone, and no studies have inquired into this aspect for a popular commercially-available artificial bone used in biomechanical studies. Sixteen fresh-frozen human femurs and five artificial femurs were obtained. Cortical specimens were mounted into a clamping system equipped with a thrust force and torque transducer. Using a CNC machine, unicortical holes were drilled in each specimen at 1000 rpm, 1250 rpm, and 1500 rpm with a 3.2 mm diameter surgical drill bit. Feed rate was 120 mm/min. Statistical significance was set at p < 0.05. Force at increasing spindle speed (1000 rpm, 1250 rpm, and 1500 rpm), respectively, showed a range for human femurs (198.4 ± 14.2 N, 180.6 ± 14.0 N, and 176.3 ± 11.2 N) and artificial femurs (87.2 ± 19.3 N, 82.2 ± 11.2 N, and 75.7 ± 8.8 N). For human femurs, force at 1000 rpm was greater than at other speeds (p ≤ 0.018). For artificial femurs, there was no speed effect on force (p ≥ 0.991). Torque at increasing spindle speed (1000 rpm, 1250 rpm, and 1500 rpm), respectively, showed a range for human femurs (186.3 ± 16.9 N·mm, 157.8 ± 16.1 N·mm, and 140.2 ± 16.4 N·mm) and artificial femurs (67.2 ± 8.4 N·mm, 61.0 ± 2.9 N·mm, and 53.3 ± 2.9 N·mm). For human femurs, torque at 1000 rpm was greater than at other speeds (p < 0.001). For artificial femurs, there was no difference in torque for 1000 rpm versus higher speeds (p ≥ 0.228), and there was only a borderline difference between the higher speeds (p = 0.046). Concerning human versus artificial femurs, their behavior was different at every speed (force, p ≤ 0.001; torque, p < 0.001). For human specimens at 1500 rpm, force and torque were linearly correlated with standardized bone mineral density (sBMD) and the T-score used to clinically categorize bone quality (R ≥ 0.56), but there was poor correlation with age at all speeds (R ≤ 0.37). These artificial bones fail to replicate force and torque in human cortical bone during surgical drilling. To date, this is the largest series of human long bones biomechanically tested for surgical drilling.

Clinical bone densitometric evaluation of the mandible in removable denture wearers dependent on the morphology of the mandibular cortex

STATEMENT OF PROBLEM

Wide normal variations have been found in the structure and density of the human skeleton, as well as of the mandible.

PURPOSE

The objective of this study was to determine whether the mandibular bone mineral density is correlated with the classification of the structure of the inferior cortex on panoramic radiographs in complete and removable partial denture wearers.

MATERIALS AND METHODS

The mandibular cortical index of 136 randomly selected complete and removable partial denture wearers was evaluated via panoramic radiographs. The criteria for the mandibular cortical index were as follow: category 1, sharp endosteal margin of the inferior cortex; category 2, semilunar defects; and category 3, thick cortical residues on endosteal margin. Forty male patients (mean age 72.7; range 56 to 84 years) and 96 female patients (mean age 69.7; range 48 to 86 years) participated. With a copper stepwedge and DenEx 2001 computer program, the mandibular bone mineral density was investigated densitometrically on dental panoramic radiographs. Four experienced observers and 6 general dental practitioners made the observations on all panoramic radiographs. All bone mineral density values were expressed in equivalents of the actual stepwedge thickness. An independent t test (alpha =.05) was used.

RESULTS

The severity of changes in the mandibular cortex was significantly related to all measured mandibular bone mineral density values (t test: P<.01). Mandibular cortical index category 3 had significantly lower bone mineral density values in all measured regions of interest. Interobserver and intraobserver agreement in mandibular cortical index assessment was excellent.

CONCLUSION

Patients having lower bone mineral density values in the mandible have much more porous cortical layer of the inferior border of the mandible.