Richard R. Tarr

Changes in tibiotalar joint contact areas following experimentally induced tibial angular deformities.

Six cadaveric legs were stripped of all soft tissue excluding the interosseous membrane and the tissues about the ankle joint and foot. Angular deformities were simulated in all planes to a maximum of 15 degrees for proximal, middle, and distal third levels following tibial resection and same-level fibular osteotomy. Anterior ankle arthrotomies allowed exposure to the tibiotalar joint so that contact area could be measured with pressure-sensitive film inserted between the tibia and talus. An angular deformity of 15 degrees or less produced no significant alteration in the contact area of the ankle joint for proximal and middle third tibial levels. Distal tibial deformities showed a dramatic change in the contact area, with as much as a 42% decrease in contact area for anterior deformities. The contact shape for distal third angular deformities of 10 degrees and 15 degrees in all planes also tended to elongate, with a shift to more lateral contact noted. Although minor degrees of angular malalignment had little effect on ankle contact for proximal and middle third levels, it would appear that distal third deformities produce a greater change in ankle joint contact; thus, fractures at the distal level should be managed to minimize the possibility of tibial malalignment.

The role of subtalar motion and ankle contact pressure changes from angular deformities of the tibia.

It is a well known entity that fractures of the tibia heal with some component of angular deformity. Ankle and subtalar joints may compensate for small degrees of angular deformities, but the exact amount of malunion that can be accepted without development of late sequalae has yet to be determined. Two recent studies from this institution have concluded that (1) contact changes at the tibiotalar joint tend to be greater with distal third tibial fracture deformities compared to proximal and middle with the ankle in neutral, 5° dorsiflexion, and 20° of plantar flexion. (2) Anterior and posterior bow deformities produced a greater change in contact area of the tibiotalar joint than with valgus or varus deformities. This phenomena may be possibly explained by the subtalar motion in the horizontal plane which averages 23°. Thus, it was the primary purpose of this paper to determine the exact role, if any, in subtalar motion on tibiotalar contact in angular deformities of the tibia. To achieve this objective the subtalar joint was transfixed thereby eliminating its perceived compensatory movement. Six cadaveric lower extremities were disarticulated at the knee joint and stripped of soft tissue preserving capsular and ligamentous structures. A custom universal joint was used to create various angulatory deformities at proximal, middle, and distal third levels of the tibia. Contact pressure across the tibiotalar joint was recorded using pressure-sensitive film and analyzed quantitatively in terms of contact area as well as pattern. The same combinations of angular deformities were then run with the subtalar joint transfixed in neutral. The results indicated that as in the two previous studies distal third deformities resulted in the greatest amount of change in ankle contact pressure area. The data also demonstrated that when subtalar motion was restricted ankle contact area decreased significantly in all planes of angulatory deformity. (1) The data collected agree with the results of two previous studies which showed that there was a decreased in total ankle contact area consistently at the distal third level with posterior angulatory deformities of the tibia. (2) By defining the resultant fracture angle and the foot axis angle a geometric explanation can be given to demonstrate a distal level fracture of the tibia has a greater effect on the ankle articulation than one more proximal. (3) The ankle joint has been shown by others to be less congruent as it moves away from its neutral position. This was found to affect and therefore cause a decrease in ankle contact area with tibial angulatory deformities. (4) The ankle joint is more adapted for weightbearing in neutral and in dorsiflexion. The anterior portion of the talar dome is probably more adapted to weightbearing than the posterior portion. This accounted for greater changes in ankle contact area during plantarflexion than in dorsiflexion. (5) The subtalar joint was found to play a very significant role in maintaining the talus in its normal relationship to the tibia. Restriction of the subtalar joint affected all deformities of the tibia as the resultant fracture angle increased. (6) The data supports Inmans concept of the subtalar joint acting as a torque transmitter and compensates for tibial varus and valgus deformities. (7) Subtalar joint restriction affected varus deformities more than valgus deformities probably due to shifting of the talar dome therefore significantly altering its normal biomechanics.

The Mechanics and Biology of Intramedullary Fracture Fixation

Intramedullary (IM) fracture fixation serves to stabilize fracture fragments and maintains alignment, while permitting motion at the fracture site during functional activities. Acting as an internal splint, the implant serves as a load-sharing device and fracture healing progresses with the formation of peripheral callus. By allowing motion of adjacent joints, rehabilitation is concurrent with treatment, and stress-shielding is thought to be minimal using these techniques. Recently, IM nails have been introduced to widen indications for their use based on variations in the cross-sectional geometry, length and shape of nails, interlocking designs, and surgical techniques. Although the most important mechanical factors in the design of IM nails are strength, stiffness, and rigidity, anatomic constraints and surgical technique limit nail variations. Closed nailing is preferred to open procedures to preserve periosteal blood supply and minimize surgical trauma adjacent to the fracture. Blood flow to the fractured bone is elevated in nailing experiments, although callus maturation is somewhat delayed. However, the end result in terms of healing was similar to that of plate fixation.

The Effect of Simulated Tibial Deformities on the Ankle Joint during the Gait Cycle

The effect of angular deformities of the tibial shaft on the area, location, and shape of the ankle joint contact during the normal extremes in the gait cycle was studied with the use of a cadaveric model. Six lower limbs were first examined radiographically and found to be free of pathology. These specimens were then stripped of soft tissues proximal to the ankle joint and had a custom-designed universal joint-plate inserted into the tibia at the proximal, middle, or distal third level. An anterior ankle arthrotomy was performed, and pressure sensitive film was inserted into the tibiotalar joint. Load was then applied with the ankle set in dorsiflexion or plantarflexion via metal wedges, and tibial deformities of 5, 10, and 15° were simulated in varus, valgus, anterior bow, and posterior bow. Contact area and location changes were noted to be of greater magnitude with proximal and distal third tibial deformities than with middle third deformities. Varus and valgus deformities showed smaller contact area changes than anterior or posterior bow deformities. Contact area changes tended to be larger in dorsiflexion compared to plantarflexion for each level and degree of tibial angulation. Posterior bow deformities at all levels resulted in greater changes in contact area and shape than other deformities. The role of subtalar compensation, stiffness of the foot-ankle complex, and geometric factors are all thought to influence the changes noted. On the basis of this experimental study it would appear that angular deformity of the tibia less than 10° would not significantly alter ankle joint contact.

Factors contributing to lysis of the femoral neck in total hip arthroplasty.

A radiographic analysis of 483 Charnley total hip arthroplasties performed from July 1970 to November 1975 demonstrated that 13% of the hips had lysis of the medial femoral neck. The findings suggest that the lysis of the medial femoral neck is significantly reduced if the femoral stem is oriented in a valgus or neutral position and with at least 5 mm of cement separating the stem of the prosthesis and the medial cortex of the femoral neck.

Clinical experiences with a titanium alloy total hip prosthesis: a posterior approach.

Using a titanium alloy femoral prosthesis (STH Zimmer) and an ultra high molecular weight polyethylene acetabular cup with a posterior surgical approach to the hip joint, 237 consecutive total hip arthroplasties in 215 patients were performed between December 1975 and May 1977. The preliminary results and early postoperative complications suggest that this system can be considered an alternative to total hip arthroplasty using other materials and surgical approaches.

IMPLANT FAILURES IN ORTHOPAEDIC SURGERY

Common orthopaedic implant failures are reviewed in the areas of total joint replacement and fracture fixation. In particular total hip and total knee arthroplasty, intertrochanteric hip fractures and long bone fractures are discussed. Excessive motion of implant bone interfaces, stress concentrations within the implant and stress shielding of bone are implicated in implant failures.