Thermal properties of polymethyl methacrylate vary depending on brand and type.

Polymethyl methacrylate (PMMA) is commonly used in orthopedic surgery and has several applications, most often for fixation of arthroplasty components. While its overall safety and tolerance are well described, less is known regarding the thermal properties of PMMA as it sets from a liquid to solid state, as well as the potential for osseous thermal necrosis. This study addresses potential variations in the setting time and maximum temperature of PMMA formulations in common use and explores the potential clinical implications of this variability. Seven commercially available formulations of PMMA that varied by brand and/or viscosity were obtained and prepared according to manufacturer's instructions. Peak temperature and duration were measured in controlled settings for each type and compared to previously described thresholds for thermal bone necrosis. Depuy SmartSet (HV), Zimmer Palacos R (HV), and Zimmer Palacos (LV) exceeded the 56°C threshold reported to potentially induce immediate osseous thermal necrosis. Additionally, Biomet Cobalt (MV) and Stryker Simplex P (MV) had lower peak temperatures but exceeded thermal necrosis thresholds due to curing duration. The lowest peak temperature was observed for Smith & Nephew Versabond (MV), which was significantly lower than all types except Depuy SmartSet (MV). Setting time was not significantly different among groups. There are significant differences in the thermal properties of PMMA formulations in current use. Selection of specific PMMA formulations represents an additional route of procedural optimization depending on the needs of the treating surgeon.

Supplementation of Lateral Locked Plating for Distal Femur Fractures: A Biomechanical Study.

OBJECTIVES: To investigate the biomechanical properties of a lateral locked plate alone or in combination with a supplemental medial plate or an intramedullary nail (IMN). METHODS: Intra-articular distal femur fractures with metaphyseal comminution (OTA/AO 33-C) were simulated with a standardized model in 28 synthetic femora and divided into 4 groups. Group I was instrumented with a 4.5-mm lateral locked distal femoral plate alone, group II with a lateral locked plate plus a low-profile precontoured 3.5-mm medial distal tibial plate, group III with a lateral locked plate plus a medial 3.5-mm reconstruction plate, and group IV with a lateral locked plate plus a retrograde IMN. Specimens were then axially loaded and cycled to failure or runout. Outcomes of interest were baseline stiffness, survivability, and cycles to failure. RESULTS: Groups III and IV have a significantly higher baseline stiffness (P < 0.001) when compared with groups I and II. Furthermore, groups III and IV had a higher max load to failure (P < 0.01) when compared with groups I and II. The survivability in groups III and IV was 71% and 100%, respectively, while no specimens in group I or II survived maximum loading. There was no significant difference between group III and IV regarding stiffness, survivability, and cycles to failure. CONCLUSION: When considering fixation for intra-articular distal femur fractures with metaphyseal comminution (OTA/AO 33-C), we found that supplementation of a lateral locked plate with a medial plate or an IMN to be biomechanically superior to lateral locked plating alone regarding stiffness, survivability, and cycles to failure. A low-profile precontoured plate did not add significantly to the construct stiffness in this study.

Long Segment Blocking Screws Increase the Stability of Retrograde Nail Fixation in Geriatric Supracondylar Femur Fractures: Eliminating the "Bell-Clapper Effect".

OBJECTIVES: To determine the change in stiffness and horizontal translations of a geriatric supracondylar femur fracture model with the addition of distal segment blocking screws versus proximal (long) segment blocking screws to the standard retrograde intramedullary nail construct. METHODS: Unstable supracondylar femur fractures (OTA/AO 33-A3) were created; all specimens were instrumented with a retrograde intramedullary nail. Specimens were divided into 2 groups (6 matched pairs per group). Group 1 compared the standard construct (1 proximal screw and 3 distal screws) to a distally augmented construct, with blocking screws placed in the distal metaphyseal segment. Group 2 compared the distally augmented construct to one in which blocking screws were placed just proximal to the fracture (long segment blocking screws). Specimens were then axially loaded and cycled to failure or run-out. RESULTS: There was no significant difference in baseline stiffness, survival through cyclic loading, stiffness after cyclic loading, or cycles to failure between femurs treated with distal blocking screws and femurs treated without blocking screws (group 1). Femurs with blocking screws in the long proximal segment had significantly greater baseline stiffness, stiffness after cyclic loading, and less horizontal translation at the fracture site (group 2). There was no difference in survival through cyclic loading or cycles to failure. CONCLUSION: Long segment blocking screws are biomechanically superior to blocking screws in the distal segment or no blocking screws initially and after cyclic loading in an unstable geriatric supracondylar femur fracture model treated with intramedullary nail. CLINICAL RELEVANCE: Surgeons may use blocking screws to aid in fracture alignment during retrograde nail fixation. In addition, the placement of long segment blocking screws can help resist failure of fixation in geriatric patients by eliminating the "Bell-clapper effect."