Christian M. Puttlitz

Biomechanical Evaluation of Arthroscopic Rotator Cuff Repairs: Double-Row Compared with Single-Row Fixation

BACKGROUND Recent studies have shown that arthroscopic rotator cuff repairs can have higher rates of failure than do open repairs. Current methods of rotator cuff repair have been limited to single-row fixation of simple and horizontal stitches, which is very different from open repairs. The objective of this study was to compare the initial cyclic loading and load-to-failure properties of double-row fixation with those of three commonly used single-row techniques. METHODS Ten paired human supraspinatus tendons were split in half, yielding four tendons per cadaver. The bone mineral content at the greater tuberosity was assessed. Four stitch configurations (two-simple, massive cuff, arthroscopic Mason-Allen, and double-row fixation) were randomized and tested on each set of tendons. Specimens were cyclically loaded between 5 and 100 N at 0.25 Hz for fifty cycles and then loaded to failure under displacement control at 1 mm/sec. Conditioning elongation, peak-to-peak elongation, ultimate tensile load, and stiffness were measured with use of a three-dimensional tracking system and compared, and the failure type (suture or anchor pull-out) was recorded. RESULTS No significant differences were found among the stitches with respect to conditioning elongation. The mean peak-to-peak elongation (and standard error of the mean) was significantly lower for the massive cuff (1.1 +/- 0.1 mm) and double-row stitches (1.1 +/- 0.1 mm) than for the arthroscopic Mason-Allen stitch (1.5 +/- 0.2 mm) (p < 0.05). The ultimate tensile load was significantly higher for double-row fixation (287 +/- 24 N) than for all of the single-row fixations (p < 0.05). Additionally, the massive cuff stitch (250 +/- 21 N) was found to have a significantly higher ultimate tensile load than the two-simple (191 +/- 18 N) and arthroscopic Mason-Allen (212 +/- 21 N) stitches (p < 0.05). No significant differences in stiffness were found among the stitches. Failure mechanisms were similar for all stitches. Rotator cuff repairs in the anterior half of the greater tuberosity had a significantly lower peak-to-peak elongation and higher ultimate tensile strength than did repairs on the posterior half. CONCLUSIONS In this in vitro cadaver study, double-row fixation had a significantly higher ultimate tensile load than the three types of single-row fixation stitches. Of the single-row fixations, the massive cuff stitch had cyclic and load-to-failure characteristics similar to the double-row fixation. Anterior repairs of the supraspinatus tendon had significantly stronger biomechanical behavior than posterior repairs.

Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus

With the potential to map mechanical properties of heterogeneous materials on a micrometer scale, there is growing interest in nanoindentation as a materials characterization technique. However, nanoindentation has been developed primarily for characterization of hard, elasto-plastic materials, and the technique has not been validated for very soft materials with moduli less than 5 MPa. The current study attempted to use nanoindentation to characterize the elastic moduli of soft, elastomeric polydimethylsiloxane (PDMS) samples (with different degrees of crosslinking) and determine the effects of adhesion on these measurements using adhesion contact mechanics models. Results indicate that nanoindentation was able to differentiate between elastic moduli on the order of hundreds of kilo-Pascals. Moreover, calculations using the classical Hertz contact model for dry and aqueous environment gave higher elastic modulus values when compared to those obtained from unconfined compression testing. These data seem to suggest that consideration of the adhesion energy at the tip-sample interface is a significantly important parameter and needs to be taken into account for consistent elastic modulus determination of soft materials by nanoindentation.

Constructs Incorporating Intralaminar C2 Screws Provide Rigid Stability for Atlantoaxial Fixation

Study Design. An in vitro biomechanical study of C1–C2 posterior fusion techniques using a cadaveric model. Objectives. To investigate the acute stability afforded across the atlantoaxial segment by a novel technique that uses intralaminar screws in C2, and to compare these results to the stability obtained using a C2 pedicle fixation technique. Summary of Background Data. There are numerous techniques available for rigidly coupling C1 and C2. It has been shown that screw techniques provide higher acute stability than wiring practices. However, many of these methods that use screw fixation in C2 can be technically difficult, especially in cases in which there is an aberrant vertebral artery course or if the C2 pedicle is not large enough to accommodate the instrumentation. A novel technique that uses intralaminar screws in C2 with C1 pedicle screws and bilateral longitudinal rods has been recently developed in an effort to overcome many of these issues. To date, there are no published reports as to whether this new technique provides equivalent (or better) fixation to the currently accepted methods. Methods. Six fresh-frozen human cadaveric cervical spines (C0–C4) were used in this study. Specimens were tested in their intact condition after destabilization via odontoidectomy, and after implantation of 3 different fixation constructs: (1) the Harms technique, 2 pedicle screws in C2, (2) a single C2 pedicle screw and a single C2 intralaminar screw, and (3) a construct having bilateral intralaminar C2 screws. Pure moment loading in flexion/extension, lateral bending, and axial rotation was applied to the occiput. Subsequent relative intervertebral rotations were determined using a 3 camera system. Range of motion for the intact, destabilized, and 3 fixation scenarios was determined, and statistical analysis was performed using one-way analysis of variance Fisher least-significant-difference post hoc test for multiple comparisons. Results. The data indicate that odontoidectomy significantly increased C1–C2 motion in flexion/extension and lateral bending. All 3 fixation techniques significantly reduced motion compared to the intact and destabilized cases. There were no statistically significant differences between the C2 intralaminar and pedicle screw techniques. Conclusions. The results clearly indicate the potential of the intralaminar screw technique to provide stability that is equivalent to methods currently used. Given the serious complications that can follow vertebral artery injury and the decreased likelihood of injury by avoiding placement of C2 pedicle screw(s) and C1–C2 transarticular screw(s), strong consideration should be given to using a construct that incorporates C2 intralaminar screw(s).

Stability analysis of craniovertebral junction fixation techniques

BACKGROUND Craniovertebral arthrodesis in the upper cervical spine is challenging because of the high degree of mobility afforded by this region. A novel method for achieving atlantoaxial fixation with use of polyaxial screws inserted bilaterally into the lateral masses of C1 and transpedicularly into C2 with longitudinal rod connection has recently been introduced. The question remains as to whether this technique provides adequate stability when extended cephalad to include the occiput. The purpose of this study was to determine the primary stability afforded by this novel construct and compare its stability with the current standard of bilateral longitudinal plates combined with C1-C2 transarticular screws. METHODS We used ten fresh-frozen human cadaveric cervical spines (C0-C4). Pure moment loads were applied to the occiput, and C4 was constrained during the testing protocol. We evaluated four conditions: (1) intact, (2) destabilized by means of complete odontoidectomy, (3) stabilization with longitudinal plates with C1-C2 transarticular screw fixation, and (4) stabilization with a posterior rod system with C1 lateral mass screws and C2 pedicle screws. Rigid-body three-dimensional rotations were detected by stereophotogrammetry by means of a three-camera system with use of marker triads. The range of motion data (C0-C2) for each fixation scenario was calculated, and a statistical analysis was performed. RESULTS Destabilization of the specimen significantly increased C0-C2 motion in both flexion-extension and lateral bending (p < 0.05). Both fixation constructs significantly reduced motion in the destabilized spine by over 90% for all motions tested (p < 0.05). No significant differences were detected between the two constructs in any of the three rotational planes. CONCLUSIONS Both hardware systems provide equivalent construct stability in the immediate postoperative period when it is critical for the eventual success of a craniovertebral arthrodesis. On the basis of this work, we believe that the decision to use either construct should be determined by clinical rather than biomechanical concerns.

Intervertebral disc replacement maintains cervical spine kinetics.

Study Design. An in vitro biomechanical study of C4–C5 intervertebral disc replacement using a cadaveric model. Objectives. To investigate the degree of motion afforded by a ball-and-socket cervical intervertebral disc prosthesis design. Summary of Background Data. Intervertebral disc prostheses designs attempt to restore or maintain cervical disc motion after anterior cervical discectomy and reduce the likelihood of accelerated degeneration in adjacent discs by maintaining normal motion at the affected disc level. Surprisingly, the actual kinetic and biomechanical effects that cervical disc arthroplasty imparts on the spine have not been widely reported. Accordingly, we investigated what effect implanting a cervical disc prosthesis has on the range of motion at the affected level as well as how it changes the coupled motion patterns at the level of implantation. Methods. Six fresh-frozen human cadaveric cervical spines (C2–C7) were used in this study. We evaluated two different spinal conditions: intact and after disc replacement at C4–C5. Compression (using the follower load concept) and pure moment loading were applied to the specimen. Range of motion was measured using an optical tracking system. Statistical differences between the intact and replaced condition range of motion was determined using analysis of variance with post hoc comparisons (&agr; = 0.05). Results. The data indicate that the intervertebral disc prosthesis approximated the intact motion in all three rotation planes at the affected level. Finally, changes in cervical coupled rotations, specifically lateral bending during axial rotation loading and axial rotation during lateral bending loading, were not statistically significant between the two tested conditions. Conclusions. Our data demonstrate that a ball-and-socket design can replicate physiologic motion at the affected and adjacent levels. More importantly, the data indicate that motion coupling, which is most dramatic in the cervical spine and plays an important biomechanical role, is maintained.

Metamaterial-based wireless strain sensors

We proposed and demonstrated metamaterial-based strain sensors that are highly sensitive to mechanical deformation. Their resonance frequency shift is correlated with the surface strain of our test material and the strain data are reported telemetrically. These metamaterial sensors are better than traditional radio-frequency (rf) structures in sensing for providing resonances with high quality factors and large transmission dips. Using split ring resonators (SRRs), we achieve lower resonance frequencies per unit area compared to other rf structures, allowing for bioimplant sensing in soft tissue (e.g., fracture healing). In 5×5 SRR architecture, our wireless sensors yield high sensitivity (109 kHz/kgf, or 5.148 kHz/microstrain) with low nonlinearity error (<200 microstrain).

Acute biomechanical and histological effects of intradiscal electrothermal therapy on human lumbar discs

Study Design. Human cadaver lumbar spines were used to assess the acute effects of intradiscal electrothermal therapy in vitro. Objective. To determine whether intradiscal electrothermal therapy produces acute changes in disc histology and motion segment stability. Summary of Background Data. Intradiscal electrothermal therapy has been introduced as an alternative for the treatment of discogenic low back pain. Several hypothesized mechanisms for the effect of intradiscal electrothermal therapy have been suggested including shrinkage of the nucleus or sealing of the anulus fibrosus by contraction of collagen fibers, and thermal ablation of sensitive nerve fibers in the outer anulus. Methods. Intradiscal electrothermal therapy was performed with the Spinecath by Oratec on 19 fresh, frozen human lumbar cadaver specimens. In a separate study, eight specimens were tested biomechanically and instrumented to map the thermal distribution, whereas five specimens were tested only biomechanically, both before and after intradiscal electrothermal therapy. Six additional specimens were heated with intradiscal electrothermal therapy, and the resulting canal was backfilled with a silicone rubber compound to allow colocalization of the catheter and anular architecture. Results. A consistent pattern of increased motion and decreased stiffness was observed. For the specimens in which only biomechanical measurements were taken, a 10% increase in the motion, on the average, at 5 Nm torque was observed after intradiscal electrothermal therapy. No apparent alteration of the anular architecture was observed around the catheter site in the intradiscal electrothermal therapy–treated discs. Conclusion. The data from this study suggest that the temperatures developed during intradiscal electrothermal therapy are insufficient to alter collagen architecture or stiffen the treated motion segment acutely.

Augmentation of a Rotator Cuff Suture Repair Using rhPDGF-BB and a Type I Bovine Collagen Matrix in an Ovine Model

Background Rotator cuff tears are a common source of shoulder pain. High rates (20%-94%) of structural failure of the repair have been attributed to multiple factors, including poor repair tissue quality and tendon-to-bone integration. Biologic augmentation using growth factors has potential to promote tendon-to-bone integration, improving the function and long-term success of the repair. One such growth factor is platelet-derived growth factor–BB (PDGF-BB), which has been shown to improve healing in tendon and bone repair models. Hypothesis Recombinant human PDGF-BB (rhPDGF-BB) combined with a highly porous type I bovine collagen matrix will improve the biomechanical function and morphologic appearance of the repair in a dose-dependent manner, relative to a suture-only control, after 12 weeks in an acute ovine model of rotator cuff repair. Study Design Controlled laboratory study. Methods An interpositional graft consisting of rhPDGF-BB and a type I collagen matrix was implanted in an ovine model of rotator cuff repair. Biomechanical and histologic analyses were performed to determine the functional and anatomic characteristics of the repair after 12 weeks. Results A significant increase in the ultimate load to failure was observed in repairs treated with 75 μg (1490.5 ± 224.5 N, P = .029) or 150 μg (1486.6 ± 229.0 N, P = .029) of rhPDGF-BB, relative to suture-only controls (910.4 ± 156.1 N) and the 500-μg rhPDGF-BB group (677.8 ± 105.9 N). The 75-μg and 150-μg rhPDGF-BB groups also exhibited increased tendon-to-bone inter-digitation histologically. No differences in inflammation or cellularity were observed among treatments. Conclusion This study demonstrated that an interpositional graft consisting of rhPDGF-BB (75 or 150 μg) and a type I collagen matrix was able to improve the biomechanical strength and anatomic appearance in an ovine model of rotator cuff repair compared to a suture-only control and the 500-μg rhPDGF-BB group. Clinical Relevance Recombinant human PDGF-BB combined with a type I collagen matrix has potential to be used to augment surgical repair of rotator cuff tears, thereby improving clinical success.

Manual In-line Stabilization Increases Pressures Applied by the Laryngoscope Blade during Direct Laryngoscopy and Orotracheal Intubation

Background:Manual in-line stabilization (MILS) is recommended during direct laryngoscopy and intubation in patients with known or suspected cervical spine instability. Because MILS impairs glottic visualization, the authors hypothesized that anesthesiologists would apply greater pressure during intubations with MILS than without. Methods:Nine anesthetized and pharmacologically paralyzed patients underwent two sequential laryngoscopies and intubations, one with MILS and one without, in random order. A transducer array along a Macintosh 3 laryngoscope blade continuously measured applied pressures, and glottic view was characterized. Results:With MILS, glottic visualization was worse in six patients, and intubation failure occurred in two of these six patients. Maximum laryngoscope pressure at best glottic view was greater with MILS than without (717 ± 339 mmHg vs. 363 ± 121 mmHg, respectively; n = 8; P = 0.023). Other measures of pressure application also indicated comparable increases with MILS. Conclusion:Pressures applied to airway tissues by the laryngoscope blade are secondarily transmitted to the cervical spine and result in cranio-cervical motion. In the presence of cervical instability, impaired glottic visualization and secondary increases in pressure application with MILS have the potential to increase pathologic cranio-cervical motion.

Flexible metamaterials for wireless strain sensing

We proposed and demonstrated flexible metamaterial-based wireless strain sensors that include arrays of split ring resonators (SRRs) to telemetrically measure strain. For these metamaterial sensors, we showed that a flexible substrate (e.g., Kapton tape) delivers greater sensitivity and a more linear response as compared to using silicon substrates. Specifically, these tape-based flexible SRR sensors exhibit a significantly improved sensitivity level of 0.292 MHz/kgf with a substantially reduced nonlinearity error of 3% for externally applied mechanical loads up to 250 kgf. These data represent a sixfold increase in sensitivity and a 16-fold reduction in error percentage.