Patrick Atkinson

Knee injuries in motor vehicle collisions: a study of the National Accident Sampling System database for the years 1979–1995

A detailed study of knee injuries recorded in the 1979-1995 National Accident Sampling System database maintained by the National Highway Traffic Safety Administration was conducted. Injuries to other body regions were also considered in order to illustrate the relative frequency of knee injuries. This study demonstrated that knee injuries constitute approximately 10% of all injuries recorded every year. However, the majority of these injuries were of low severity (i.e. contusions, abrasions, lacerations) with an abbreviated injury score (AIS) of 1. Most knee injuries occurred following a frontal collision with no intrusion. The study also indicated most knee fractures occur in crashes where the vehicle velocity differences (deltaVs) were less than 45 kmph, with some occurring at deltaVs as low as 10 kmph. Serious non-fracture knee injuries (i.e. ligament tears) rated AIS 2 accounted for 20 out of every 1000 injuries and predominantly occurred at deltaVs below 25 kmph. In this study it was noted that women were more likely to experience a knee contusion than men. This study further suggests that knee impact scenarios have remained relatively constant over the years as the knee injury rates showed little variation. The rate of lap and shoulder belt use was lower in occupants who experienced a knee injury vs. the rate in the overall database and airbags were present in only a small number of cases. As this study largely included only vehicles without airbags it provides a good baseline for analysis of the influence of the airbag on knee injury trends in the future.

Volume and surface area study of tobramycin-polymethylmethacrylate beads.

Polymethylmethacrylate bone cement beads impregnated with antibiotic are a common treatment for patients with persistent articular joint infections or osteomyelitis. They also are used as a prophylaxis for infection in patients with large soft tissue wounds. The current study was designed to evaluate the relationship between bead geometry and elution of the antibiotic tobramycin by methodically varying the shape of the beads for a given set of volumes. Beads of five shapes (spherical to ovoid) and two volumes were prepared and studied. Only 0.9% to 3.3% of the total amount of tobramycin present actually eluted from the beads in a 96-hour period and of this amount, approximately ⅓ eluted within the first 4 hours. The elution mass data indicate the benefit of numerous, small and elliptically shaped beads for maximal antibiotic availability. Additionally, a mathematical model is presented that describes these findings and can be used to predict tobramycin delivery rates from bone cement beads. This model assumes that the antibiotic is delivered through two mechanisms: fast dissolution of tobramycin initially adhering to the bead surface and slow release by diffusion through the polymer. The results generate diffusion coefficients for tobramycin in polymethylmethacrylate bone cement on the order of 2 × 10−11 cm2/s.

Injuries produced by blunt trauma to the human patellofemoral joint vary with flexion angle of the knee

Patellofemoral joint impact trauma during car accidents, sporting activities, and falls can produce acute gross fracture of bone, microfracture of bone, and soft tissue injury. Field studies of car accidents, however, show that most patellofemoral traumas are classified as ‘subfracture’ level injuries. While experimental studies have shown that the influence of flexion angle at impact is not well understood, flexion angle may influence injury location and severity. In the current study, 18 pairs of isolated human cadaver knees were subjected to blunt impact at flexion angles of 60°, 90°, or 120°. One knee from each cadaver was sequentially impacted until gross fracture of bone was produced. The contralateral knee was subjected to a single, subfracture impact at 45% of the impact energy producing fracture in the first knee. The fracture experiments produced gross fracture of the patella and femoral condyles with the fracture plane positioned largely within the region of patellofemoral contact. The fracture location and character changed with flexion angle; at higher flexion angles the proximal pole of the patella and the femoral condyles were more susceptible to injury. For the 90° flexion angle, the patella was fractured centrally, while at 60° the distal pole fractured transversely at the insertion of the patellar tendon. In addition, the load magnitude required to produce fracture increased with flexion angle. In the ‘subfracture’ knees, injuries were documented for all flexion angles: occult microfractures of the subchondral and trabecular bone and fissures of the articular surface. Similar to the fracture‐level experiments, the injuries coincided with the patellofemoral contact region. These data show that knee flexion angle plays an important role in impact related knee trauma. Such data may be useful in the clinical setting, as well as in the design of injury prevention strategies.

A method to increase the sensitive range of pressure sensitive film

Pressure-sensitive film is frequently used in biomechanics to document intra- and extra-articular contact pressures. This often involves the contact of two surfaces of varying curvature producing non-uniform pressure distributions. The purpose of this study was to determine the feasibility of using multiple films in such experiments to yield accurate pressure and contact area data. A composite arrangement of film was dynamically loaded using cylindrical indenters of five radii. An analytical model of each indentation was constructed to provide a standard for error analysis. The study showed that several ranges of pressure sensitive film can be used simultaneously to accurately transduce contact pressures arising from loading scenarios that produce contact pressure gradients and contact pressures that involve suprathreshold loading of a given film range.

THE INFLUENCE OF IMPACT INTERFACE ON HUMAN KNEE INJURY: IMPLICATIONS FOR INSTRUMENT PANEL DESIGN AND THE LOWER EXTREMITY INJURY CRITERION

A single impact at 27 J of energy with a rigid interface was delivered to one knee of isolated joint preparations of six cadavers resulting in an average peak load of 5 kN. Contralateral knee were impacted with a padded interface at an additional level of energy at approximately the same load. Math modeling of the patellae showed reduced tensile and shear stresses in the bone with padding. Increasing contact area over the knee reduces stresses in the bone and prevents both gross bone fracture and bone and cartilage microdamage. Language: en

Impact Responses of the Flexed Human Knee Using a Deformable Impact Interface

Blunt impact trauma to the patellofemoral joint during car accidents, sporting activities, and falls can produce a range of injuries to the knee joint, including gross bone fracture, soft tissue injury, and/or microinjuries to bone and soft tissue. Currently, the only well-established knee injury criterion applies to knee impacts suffered during car accidents. This criterion is based solely on the peak impact load delivered to seated cadavers having a single knee flexion angle. More recent studies, however, suggest that the injury potential, its location, and the characteristics of the damage are also a function of knee flexion angle and the stiffness of the impacting structure. For example, at low flexion angles, fractures of the distal patella are common with a rigid impact interface, while at high flexion angles splitting of the femoral condyles is more evident. Low stiffness impact surfaces have been previously shown to distribute impact loads over the anterior surface of the patella to help mitigate gross and microscopic injuries in the 90 deg flexed knee. The objective of the current study was to determine if a deformable impact interface would just as effectively mitigate gross and microscopic injuries to the knee at various flexion angles. Paired experiments were conducted on contralateral knees of 18 human cadavers at three flexion angles (60, 90, 120 deg). One knee was subjected to a fracture level impact experiment with a rigid impactor, and the opposite knee was impacted with a deformable interface (3.3 MPa crush strength honeycomb material) to the same load. This (deformable) impact interface was effective at mitigating gross bone fractures at approximately 5 kN at all flexion angles, but the frequency of split fracture of the femoral condyles may not have been significantly reduced at 120 deg flexion. On the other hand, this deformable interface was not effective in mitigating microscopic injuries observed for all knee flexion angles. These new data, in concert with the existing literature, suggest the chosen impact interface was not optimal for knee injury protection in that fracture and other minor injuries were still produced. For example, in 18 cadavers a total of 20 gross fractures and 20 subfracture injuries were produced with a rigid interface and 5 gross fractures and 21 subfracture injuries with the deformable interface selected for the current study. Additional studies will be needed to optimize the knee impact interface for protection against gross and microscopic injuries to the knee.

Design and assessment of a wrapped cylindrical Ca-P AZ31 Mg alloy for critical-size ulna defect repair†

Recently, magnesium has been investigated as a promising bioresorbable orthopedic biomaterial. Its mechanical properties are very similar to natural bone, making it appropriate for load-bearing orthopedic fracture repair applications. However, significant hurdles remain regarding the design of practical implants and methods to control degradation and enhance biocompatibility. Although attempts have been made to hinder magnesiums rapid corrosion via alloying and coating, these studies have used solid monoliths. In an effort to reduce the amount of alloy used for implantation in a shape that mimics cortical bone shape, this study used a thin sheet of Mg AZ31 which was rolled into hollow cylindrical scaffolds. The scaffold was coated with different amounts of Ca-P; this implant demonstrated slowed corrosion in simulated body fluid (SBF) as well as enhanced biocompatibility for mesenchymal stem cells (MSC). In vivo implantation of magnesium alloy scaffold adjacent to the rat femur showed significant biointegration with further deposition of complex Mg-Ca phosphates/carbonates typical of natural bone. Finally, the implant was placed in a critical-size ulna defect in live rabbits, which lead to radiographic union and partial restoration of biomechanical strength in the defect. This study demonstrated that a thin sheet of coated Mg alloy that was spirally wrapped wound be a promising orthopedic biomaterial for bone repair.

Blunt Injuries to the Patellofemoral Joint Resulting From Transarticular Loading Are Influenced by Impactor Energy and Mass

Various impact models have been used to study the injury mechanics of blunt trauma to diarthrodial joints. The current study was designed to study the relationship between impactor energy and mass on impact biomechanics and injury modalities for a specific test condition and protocol. A total of 48 isolated canine knees were impacted once with one of three free flight inertial masses (0.7, 1.5, or 4.8 kg) at one of three energy levels (2, 11, 22 J). Joint impact biomechanics (peak load, loading rate, contact area) generally increased with increasing energy. Injuries were typically more frequent and more severe with the larger mass at each energy level. Histological analyses of the patellae revealed cartilage injuries at low energy with deep injuries in underlying bone at higher energies.

DEVELOPMENT OF INJURY CRITERIA FOR HUMAN SURROGATES TO ADDRESS CURRENT TRENDS IN KNEE-TO-INSTRUMENT PANEL INJURIES

Lower extremity injuries during car accidents are common; the lower extremities are typically the first point of contact between the occupant and the car interior. Lower extremity injuries are not normally life threatening, but can represent a large societal burden through treatment costs, lost work days, and reduced quality of life. The purpose of this research was to study injuries of the knee and propose a methodology to prevent future knee injuries. Data from the National Accident Sampling System (NASS) showed that 10% of all injuries were to the knee, second only to head and neck injuries. Most knee injuries are a result of knee-to-instrument panel and subfracture injuries were most common, followed by gross fracture injuries. Cadaver data show that increasing the contact area for a given contact force over the knee greatly reduces acute injury in fracture and subfracture experiments. However, cadaver force-area data cannot be applied to the Hybrid III dummy, which is the most used human surrogate in car crash simulations. This study also sought to develop a transformation of the cadaver contact force-area relationship to the dummy. Numerous experiments were conducted on the dummy to establish a comparison with companion experiments conducted on cadavers. Data points representing a 50% risk of gross fracture were calculated for the cadaver and transformed into the dummy response to yield data directly relevant to sled testing with dummies. Several sled tests were run using an idealized instrument panel to show the utility of the data in predicting joint injury for depowered air bags and various restraint scenarios. Mathematical models were used to show a theoretical scenario in which load and area could be estimated without the need for sled testing. This study shows that a simple measure of knee contact load and area could be used to predict injuries in the cadaver knee from blunt insult via dummy test data and hopefully provide increased knee injury protection for car occupants.

Lower extremity injuries in motor vehicle collisions: a survey of NASS 1997-1999

Lower extremity injury is common in front seated occupants of motor vehicles involved in collisions. The recent introduction of air bags, while intended to reduce injury to vital body regions, namely the head and chest, may influence the incidence of lower limb injury. The current study examined lower extremity injuries recorded in the National Automotive Sampling System in order to identify injury rates and trends. NASS data from 1995-1999 suggests drivers sustain a lower extremity injury in 35% of collisions. However, the rate of moderate-to-severe injury is low occurring in approximately 11% of front seat occupants. The most common site for moderate-to-serious injury was the foot/ankle. The thigh was most frequently the site of serious lower extremity injuries, with the majority of these injuries due to instrument panel contact. Air bag deployment does not appear to alter the risk of lower extremity injury, while seat belt use tends to slightly decrease the risk.