Limited role of direct exchange arthroplasty in the treatment of infected total hip replacements.

A literature review was performed to determine when direct exchange was most likely to be successful. Twelve reports provided outcome data on infected hip replacements treated with direct exchange. The average duration of followup was 4.8 years, but the range was broad (0.1-17.1 years). Of the 1,299 infected hip replacements treated with direct exchange, 1,077 (83%) were thought to be free of infection at the last followup. Antibiotic-impregnated bone cement was used in 1,282 of the cases (99%). There was wide variability in the duration of parenteral antibiotic therapy, ranging from just 24 hours to as many as 8 weeks. In some cases, no oral antibiotics ever were given, whereas in others, oral antibiotics were given for as many as 8 months after parenteral therapy. Factors associated with a successful direct exchange included: (1) absence of wound complications after the initial total hip replacement; (2) good general health of the patient; (3) methicillin-sensitive Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus species; and (4) an organism that was sensitive to the antibiotic mixed into the bone cement. Factors associated with failure included: (1) polymicrobial infection; (2) gram-negative organisms, especially Pseudomonas species; and (3) certain gram-positive organisms such as methicillin-resistant Staphylococcus epidermidis and Group D Streptococcus. Methicillin-resistant organisms have become more common. Many current revision surgical techniques use cementless implants. Fixation without any cement (no depot antibiotics) may be a contraindication to direct exchange. Additionally, there essentially are no data on the use of bone graft in association with direct exchange. For these reasons, the indications for direct exchange are limited.

The John Charnley Award. Wear is a function of use, not time.

Polyethylene wear (linear penetration) in 37 hip replacements was assessed from digital images using a validated two-dimensional, edge detection-based computer algorithm. Patient activity was assessed with a pedometer, a step activity monitor and a simple visual analog scale. Joint use was related to wear at the 90% confidence level. Without three recognized outliers, wear was highly correlated to use. The visual analog scale activity rating was significantly related to wear for the 24 hip replacements with standard polyethylene. Univariate regression analysis indicated that male gender, height, weight (which were both highly correlated to male gender) and hip center of rotation were significantly correlated to wear. Multivariate regression analysis indicated that male gender, femoral off-set, and Hylamer were significantly correlated to wear. Based on the wear and activity data from the 24 hip replacements with standard polyethylene, the average volumetric wear rate per million cycles with a 70 kg patient weight was 30 mm3. This unique in vivo result can be considered a target wear rate for standard polyethylene in hip simulator studies.

Maximal contraction lessens impact response in a muscle contusion model.

The effect of muscle contraction on a contusion injury model was studied in the gastrocnemius muscle of anesthetized rats. Both limbs of 18 rats received a contusion injury with a blunt non-penetrating impact. One hind limb was relaxed during impact and the other was electrically stimulated to tetanic contraction. The impact was produced using a drop-mass technique (mass = 171 g, height = 101 cm, spherical radius of impactor tip = 6.4 mm). The impact response was determined by sampling (10 kHz) the transmitted impact force and the displacement of the impactor. In a subgroup of nine rats, the severity of the contusion injury was measured by recording contractile tension in twitch and tetanus within two hours of injury. We found that the peak impact force was significantly less (p < 0.01), while the peak impact displacement was significantly greater (p < 0.01) in the contracted limb. Correspondingly, the impact stiffness of the contracted limb was significantly less (p < 0.01) than the impact stiffness in the relaxed limb. Both impacts produced significant injuries relative to an uninjured control group. The tetanic tension (31 +/- 4 N) generated by the muscles that were contracted during impact was significantly (p < 0.03) greater than that generated by the muscles that were relaxed during impact (27 +/- 4 N). The findings from this specific model indicate that the impact response of the limbs with relaxed muscle was dominated by the underlying bone, while maximally contracted muscle decreased the influence of the bone and lessened the impact response. Maximally contracted muscle was not more susceptible to injury and may act as protective mechanism against some impacts.