Sanjiv H. Naidu

Percutaneous pinning of distal radius fractures: A biomechanical study***

Mechanical testing of extra-articular distal radius fractures stabilized with percutaneous pins was performed. Twelve fresh-frozen unembalmed radii stripped of all soft tissues were used. An osteotomy with dorsal comminution was made 2.5 cm proximal to the radial styloid. Three pin sizes and four pin configurations were used to fix the fracture, and the constructs were mechanically tested. Results show that at least a .062-inch pin was required for any significant changes in rigidity of the four pin configurations tested to be discerned. Cross-pinning with two radial styloid pins and placement of a pin from the ulnar corner of the radius as the most rigid construct in both torsion and cantilever bending.

Influence of metacarpophalangeal joint position on basal joint-loading in the thumb.

Background: Conventional wisdom holds that hyperextension of the metacarpophalangeal joint of the thumb is secondary to degenerative subluxation of the trapeziometacarpal joint as occurs in osteoarthritis. We propose that a hypermobile metacarpophalangeal joint may have a causative role in the development of primary osteoarthritis at the base of the thumb by concentrating forces on the palmar aspect of the trapeziometacarpal joint. Methods: Twenty fresh-frozen cadaveric forearm specimens were obtained post mortem from donors with no history of connective-tissue disease. Each specimen was categorized by its passive range of metacarpophalangeal joint motion. Testing was conducted with Fuji ultra-low-pressure‐sensitive film while the hand was in the lateral-pinch mode with the metacarpophalangeal joint in each of the following positions: unrestrained, pinned in neutral, pinned in 30° of flexion, and pinned in maximal hyperextension. Quantitative analysis of the trapezial contact surface at each of the metacarpophalangeal joint positions was performed, and the center of pressure was determined. Each specimen was then classified according to the extent of arthritic disease (nonarthritic, moderately arthritic, or affected by end-stage arthritis). Results: In specimens affected by end-stage osteoarthritis, the center of pressure on the trapeziometacarpal joint moved dorsally by 56.8% of the length of the trapezial surface with metacarpophalangeal joint flexions of 30° (p < 0.01), whereas the corresponding values were 28.2% and 40.9% in the hyperextended and neutral metacarpophalangeal joint positions, respectively. In specimens with moderate osteoarthritis, 30° of metacarpophalangeal joint flexion also produced the most dorsal trapeziometacarpal center of pressure (44.8%); however, this center of pressure was not significantly different from the centers of pressure at the other metacarpophalangeal joint positions. In nonarthritic specimens, the center of pressure was again significantly more dorsal with metacarpophalangeal joint flexion of 30° than it was at the other positions (p < 0.01). Conclusion: Metacarpophalangeal joint flexion effectively unloaded the most palmar surfaces of the trapeziometacarpal joint regardless of the presence or severity of arthritic disease in this joint. Clinical Relevance: The presence of hyperextension laxity of the metacarpophalangeal joint may identify individuals who are predisposed to the development of arthritis of the trapeziometacarpal joint; such individuals might benefit from early intervention to stabilize the metacarpophalangeal joint and thus to retard the natural progression of osteoarthritic disease at the base of the thumb. Likewise, in symptomatic patients with a hypermobile metacarpophalangeal joint, fixation of the metacarpophalangeal joint in flexion by either splinting or surgical stabilization may alleviate basal joint symptoms by redirecting trapeziometacarpal joint forces away from the palmar compartment and onto the healthier dorsal aspect of the joint.

Percutaneous Pinning of Proximal Humerus Fractures: A Biomechanical Study

Mechanical testing of two-part surgical neck fractures fixed with four different pin configurations was performed. Ten fresh, frozen, unembalmed humeri stripped of all soft tissues were used; the surgical neck was osteotomized perpendicular to the humerus long axis. Terminally threaded 2.5-mm AO pins were used to fix the fracture. Humeri then were tested in both torsion and bending on a custom-made jig using Instron 1331 to assess the rigidity of pinning constructs. In torsion, two lateral pin construct was significantly less rigid than all other pin configurations. The addition of an anterior pin to two lateral pins did not increase bending rigidity, but significantly increased torsional stiffness. The addition of two bicortical tuberosity pins or two bicortical tuberosity pins and one anterior pin to two lateral pins significantly increased rotational and bending rigidity. Results confirm clinical data, and the authors conclude that multiplanar pins are needed to augment torsional stiffness, and that the addition of two bicortical tuberosity pins enhances bending rigidity.

External Fixation of Distal Radius Fractures: Effect of Distraction and Duration

PURPOSE To assess the effects of the amount of distraction across the wrist and the duration of fixator placement on the clinical outcome of patients with distal radius fractures treated with external fixation. METHODS A total of 42 patients with closed distal radius fractures were treated with a spanning external fixator plus supplementary percutaneous K-wires over a 6-year period. All fractures were extra-articular (A type) or simple intra-articular (C type). Twenty-four of these patients were evaluated retrospectively for clinical and radiographic outcomes at an average follow-up time of 22 months (range, 4-49 months). The amount of distraction attained by the fixator was determined by measuring the carpal height ratio on plain radiographs. Wrist and forearm range of motion were recorded, as well as grip and pinch strength. Standard radiographs were taken to evaluate healing and bony alignment. RESULTS Using the Gartland Werley classification, there were 11 excellent, 10 good, and 3 fair results. Statistical analysis indicated that a higher carpal height ratio at the initial reduction positively correlated (p = .041) with an excellent outcome. Duration of external fixation did not have a significant impact on the final outcome within the parameters studied (p = .891). Average wrist range of motion at follow-up was as follows: flexion, 54.1 degrees (75% of the contralateral side); extension, 59.0 degrees (78%); radial deviation, 18.0 degrees (85%); ulnar deviation, 22 degrees (73%); pronation, 79.0 degrees (95%); and supination, 76.6 degrees (93%). None of the individual components of range of motion were negatively correlated with higher carpal height ratio at fixator application or duration of fixation, within the parameters studied. CONCLUSIONS Moderately increased distraction of the carpus at the initial fracture reduction is correlated with improved clinical outcome and does not have an adverse affect on subsequent wrist range of motion. TYPE OF STUDY/LEVEL OF EVIDENCE Therapeutic IV.

Pre- and postimplantation dynamic mechanical properties of silastic HP-100 finger joints

Dynamic mechanical properties of Silastic HP-100 finger joint implants were studied. Sixteen sections of unimplanted and 14 sections of retrieved implants were analyzed with standard thermal analysis techniques. Results show that implantation does not alter the viscoelastic properties of Silastic HP-100 elastomers. The authors conclude that fracture of Silastic HP-100 implants cannot be attributed to changes in rheologic properties of the implanted elastomer.

Radiation-Induced Physical Changes in UHMWPE Implant Components

Post-irradiation aging of ultra-high molecular weight polyethylene (UHMWPE) is not well understood. Retrieval studies and in vitro aged specimens have shown oxidative changes along with increases in crystallinity. Critical analysis and review of the polymer science and polymer physics literature shows that while oxidation may be important during the first year post-irradiation, subsequent aging occurs because of initial gamma radiation-induced chain scission leading to eventual isothermal crystallization of polymer chains in the amorphous regions of the UHMWPE bulk. Mechanical properties of aged UHMWPE are not as yet clear and, until such data become available, gamma irradiation sterilization must be used with caution.

Modular ulnar head decoupling strength: a biomechanical study.

PURPOSE Knowing the assembly and disassembly forces of modular ulnar head implants will be invaluable to the surgeon faced with the challenge of either revising or extracting the implant. Our goal in this study was to evaluate the decoupling strength of the modular ulnar head implant as a function of assembly impaction force for the 3 most commonly used combinations of modular ulnar head implants. METHODS Assembly forces and axial decoupling strength for 3 combinations of modular ulnar head implants were measured on an Instron 4206 instrument. Correlations between Morse taper decoupling strength and assembly forces were investigated. RESULTS The cobalt chrome head-cobalt chrome stem Morse taper couple showed the most predictable correlation with impaction assembly force, followed by cobalt chrome head-titanium stem couple and ceramic head-titanium couple, respectively. CONCLUSIONS Peak assembly impaction blow at the time of implantation determines the ultimate decoupling strength of the modular ulnar head Morse taper for all 3 material combinations studied. Prosthesis-specific regression curves will permit the surgeon to estimate decoupling force as a function of impact force.

Modular Ulnar Head Decoupling Force: Case Report

Cobalt-chrome modular distal ulnar head replacement arthroplasty is a surgical option to restore stability to the distal radioulnar joint rendered unstable by hemi-resection arthroplasty or a total resection arthroplasty. However, the revision of dislocated modular cobalt-chrome ulnar head implants may pose an important intraoperative challenge. The Morse-taper disassembly force of modular ulnar head implants is not available in the current published literature. We present a case in which tremendous difficulty was encountered while revising a dislocated modular cobalt-chrome distal ulnar head implant. The mean Morse-taper disassembly force of the retrieved modular cobalt-chrome implant was 2958 N +/- 1272. At nearly 4.5 times the average body weight, the modular ulnar head Morse-taper disassembly strength presented a formidable force to overcome intraoperatively.

Isolated digital swelling as the initial presentation of juvenile rheumatoid arthritis

Juvenile rheumatoid arthritis (JRA) is an inflammatory condition that affects children under 16 years of age. The cases of 7 patients with isolated finger swelling as the initial manifestation of JRA are reviewed. In these children, the diagnosis was significantly delayed (t-test, p < .0076) for up to 14 months (mean, 8 months) when compared to a mean of 3.4 months in the JRA patients with a typical onset. Of the patients with the initial presentation of isolated digital swelling, +/7 (57%) had disease that became polyarticular, whereas only 15% of the patients with the initial presentation of large-joint disease experienced progression to polyarticular disease (Fishers exact test, p < .0307). This presentation of JRA should be recognized so that appropriate management can be instituted promptly.

Isoelastic Distal Ulnar Head Prosthesis: An In Vitro Joint Simulator Study

PURPOSE A prosthetic ulnar head that matches the mechanical properties of the sigmoid notch of the radius may produce a superior clinical outcome. In this study, we investigated the feasibility of an ultrahigh-molecular-weight polyethylene (UHMWPE) ulnar head prosthesis as a bearing material and compared the tribology (wear properties) with that of a cobalt chrome (CoCr) distal ulnar replacement. METHODS We compared the friction properties, termed lubricity of the UHMWPE ulnar head prosthesis and polished CoCr ulnar head in an in vitro joint simulator with physiologic saline maintained at body temperature. Wear of the UHMWPE ulnar head and polished CoCr ulnar head against a synthetic bone counterface for 1 million cycles was simulated in the in vitro joint simulator. Finally, we studied the fatigue of the UHMWPE prosthesis for 3.1 million cycles of cyclic loading. Both the fatigued and nonfatigued surface were studied with thermal analysis to characterize the UHMWPE prosthesis. RESULTS The dynamic coefficient of friction for the UHMWPE prosthesis was 0.087 +/- 0.016 and was not significantly different from the polished CoCr coefficient. The synthetic counterface bone wear was 45 times less for the UHMWPE ulnar head prosthesis compared with the polished CoCr head. Both the thermal properties and the viscoelastic moduli for the fatigued UHMWPE remained unchanged compared with unloaded specimens. CONCLUSIONS Ultrahigh-molecular-weight polyethylene is a low-friction material with stable fatigue properties. In addition, in light of its low modulus of elasticity, the UHMWPE ulnar head prosthesis demonstrated low synthetic bone counterface wear compared with the polished cobalt chrome alloy prosthesis. In vitro joint simulation showed that there is a potential role for the UHMWPE ulnar head prosthesis in distal radioulnar joint prosthetic arthroplasty.