The effect of femoral component head size on posterior dislocation of the artificial hip joint.

BACKGROUND: Posterior dislocation continues to be a relatively common complication following total hip arthroplasty. In addition to technical and patient-associated factors, prosthetic features have also been shown to influence stability of the artificial hip joint. In this study, a dynamic model of the artificial hip joint was used to examine the influence of the size of the head of the femoral component on the range of motion prior to impingement and posterior dislocation following total hip replacement. METHODS: Six fresh cadaveric specimens were dissected, and an uncemented total hip prosthesis was implanted in each. Each specimen was mounted in a mechanical testing machine and loaded with use of a system of seven cables attached to the femur and pelvis that simulated the action of the major muscle groups crossing the hip joint. The hip was taken through a range of motion similar to that experienced when rising from a seated position. The three-dimensional position of the femur at the points of impingement and dislocation was recorded electronically. The range of joint motion was tested with prosthetic femoral heads of four different diameters (twenty-two, twenty-six, twenty-eight, and thirty-two millimeters). RESULTS: Significant associations were noted between the femoral head size and the degree of flexion at dislocation in ten (p = 0.001), twenty (p < 0.001), and thirty (p = 0.003) degrees of adduction. Increasing the femoral head size from twenty-two to twenty-eight millimeters increased the range of flexion by an average of 5.6 degrees prior to impingement and by an average of 7.6 degrees prior to posterior dislocation; however, increasing the head size from twenty-eight to thirty-two millimeters did not lead to more significant improvement in the range of joint motion. The site of impingement prior to dislocation varied with the size of the femoral head. With a twenty-two-millimeter head, impingement occurred between the neck of the femoral prosthesis and the acetabular liner, whereas with a thirty-two-millimeter head, impingement most frequently occurred between the osseous femur and the pelvis. CONCLUSIONS: With the particular prosthesis that was tested, increasing the diameter of the femoral head component increased the range of motion prior to impingement and dislocation, decreased the prevalence of prosthetic impingement, and increased the prevalence of osseous impingement. CLINICAL RELEVANCE: These results suggest that femoral heads with a twenty-eight-millimeter diameter increase the range of motion after total hip replacement. This may be beneficial when additional factors compromising joint stability are encountered.

The morphology of the femur in developmental dysplasia of the hip.

We studied the morphometry of 35 femora from 31 female patients with developmental dysplasia of the hip (DDH) and another 15 from 15 age- and sex-matched control patients using CT and three-dimensional computer reconstruction models. According to the classification of Crowe et al 15 of the dysplastic hips were graded as class I (less than 50% subluxation), ten as class I/III (50% to 100% subluxation) and ten as class IV (more than 100% subluxation). The femora with DDH had 10 to 14 degrees more anteversion than the control group independent of the degree of subluxation of the hip. In even the most mildly dysplastic joints, the femur had a smaller and more anteverted canal than the normal control. With increased subluxation, additional abnormalities were observed in the size and position of the femoral head. Femora from dislocated joints had a short, anteverted neck associated with a smaller, narrower, and straighter canal than femora of classes I and II/III or the normal control group. We suggest that when total hip replacement is performed in the patient with DDH, the femoral prosthesis should be chosen on the basis of the severity of the subluxation and the degree of anteversion of each individual femur.

Proximal and distal femoral centralizers in modern cemented hip arthroplasty.

Third generation cementing techniques using intramedullary restrictors, low porosity cement with pressurization, lavage, and cement-stem bond enhancement do not prevent implant malalignment and inadequate cement mantle thickness. This has led to the development of modular proximal and distal centralizers to control the alignment of the femoral component and maintain an adequate thickness of the cement, thereby theoretically decreasing the rate of aseptic loosening. A retrospective analysis was performed of 100 primary cemented centralized femoral components. At an average followup of 5.7 years (range, 4-8 years), the average Harris Hip Score was 90. There were no cases of aseptic loosening, osteolysis, or impending failure. Ninety-one percent of femoral stems were implanted with satisfactory alignment with an optimal cement thickness. However, six distal centralizers and one proximal centralizer fractured at the time of insertion and voids frequently were seen in and around the distal centralizer. Although centralizers improve prosthesis alignment and cement mantle thickness, the long term effects of centralizer fracture and distal cement voids need to be observed to determine if centralizers improve previous implant survival.

Pressurization and centralization enhance the quality and reproducibility of cement mantles.

Cementing technique has a profound influence on the incidence of aseptic loosening of total hip replacements. Two specific measures that seem to have the greatest impact on the longevity of cemented femoral stems are pressurization of cement and control of mantle thickness, typically through the use of modular centralizing devices attached to the tip of the prosthesis. Two laboratory studies are presented that examine the success of these measures in clinical practice. In the first study, the performance of five designs of intramedullary plugs in resisting migration during pressurization of cement was evaluated in human anatomic specimen femurs. Profound differences were observed between the performance of the different plug designs. In canals larger than 12 to 14 mm, most commercial devices failed to resist pressures greater than 30 to 40 pounds per square inch. Overall, it was estimated that between 6% to 76% of these devices would fail to resist cement pressures of 50 pounds per square inch in clinical practice. The second study examined the role of distal centralizers in the accumulation of air bubbles around the distal tip of the prosthesis during insertion of the stem into the femur. Acrylic replicas of a femoral stem were implanted in cavities simulating the femoral canal. Colored dyes, present within the cement, revealed the complex patterns of cement flow. It was shown that cement, dragged from the top of the femur, forms a thin layer that covers the entire surface of the prosthesis and the distal centralizer. Significant voids were present behind the trailing edges of the distal centralizer in 42% of the cases examined. These studies show that improvements in intramedullary plugs and stem centralizers are needed to increase the reproducibility of cement technique in total hip replacement.

Anatomic alignment of the patellar groove.

The variability in alignment of the natural patellar groove was determined about various anatomic axes of the femur, using 3 plane radiographs and electronic digitization. After the patellar groove was identified and marked on 15 anatomic specimen femurs, radiographs were taken in the coronal, sagittal, and transverse planes so that principal anatomic axes could be outlined. Through electronic digitization, a 3-dimensional representation of the patellar groove was constructed about the distal anatomic axis, mechanical axis, transepicondylar axes, and transcondylar axes. Regarding these 4 principal anatomic axes, the variability in orientation of the patellar groove was profound in both coronal and transverse planes, typically involving a range of 11 degrees to 16 degrees about the mean. The average orientation most closely approximated the perpendicular to the transepicondylar axis in the coronal plane; however, the range varied extensively. None of the anatomic axes tested proved reliable as a reference axis for proper position of the patellar groove, and this study shows that the orientation of the natural patellar groove is more variable than previously suspected. The failure of femoral components to accommodate this variability may explain many complications associated with the patellar component in total knee arthroplasty.

Reconstitution of the patellar tendon donor site after graft harvest.

Little is known about the fate of the donor site after the central 1/3 of the patellar tendon is harvested for anterior cruciate ligament reconstruction. This study evaluated the donor site in the patellar tendon at various times after a graft from the central 1/3 of the patellar tendon was harvested. Fourteen patients were studied with magnetic resonance images taken from 6 weeks to 2 years after anterior cruciate ligament reconstruction. Axial, sagittal, and coronal views of the patellar tendon were obtained. A second group of 8 patients who had previous anterior cruciate ligament reconstruction were returned to the operating room for subsequent procedures on the affected knee. These procedures were performed 2 to 24 months after the original reconstruction. An open biopsy was obtained from the donor site in the patellar tendon. On magnetic resonance images, the size of the defect and the intensity of the signal in the central 1/3 of the tendon decreased with time from surgery. At 2 years, the defect was indistinguishable from normal tendon. Histologically, the scar in the defect progressively matured with time, becoming nearly identical to normal tendon at 2 years.

Distal stem design and the torsional stability of cementless femoral stems.

In cementless hip arthroplasty, the fit between the implant and the endosteal cavity is a critical determinant of implant stability. Although cementless implants may be stabilized through proximal fit within the metaphysis, many surgeons rely on diaphyseal fixation to provide the necessary resistance to rotational forces, especially in revision hip arthroplasty. The cross-sectional design of the femoral stem at the level of the femoral isthmus was investigated with respect to its effect on the rotational stability of the bone-stem interface. Four cross-sectional designs--a fluted stem, a finned stem, a porous-coated stem, and a slotted fluted stem--were implanted in 12 cadaveric femurs and loaded in torsion. A knurled stem, cemented into each specimen at the conclusion of testing, acted as a control stem. The torque required to cause 100 microns of displacement at the bone stem-interface ranged from 13.7 +/- 0.8 N-m with the porous-coated design to 30.1 +/- 3.7 N-m with the fluted design (P < .0001). Intermediate values of 19.5 +/- 1.4 and 19.9 +/- 2.3 N-m were observed with the finned and slotted fluted designs, respectively. In all of the cemented control stems, failure occurred at the bone-cement interface at an average torque of 34.0 +/- 3.0 N-m. Statistical analysis demonstrated that the porous-coated, finned, and slotted fluted designs were all significantly weaker in torsion than the cemented control stem; however, there was no significant difference between the torsional resistance of the solid fluted (unslotted) and cemented stems.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors influencing pressurization of the femoral canal during cemented total hip arthroplasty.

Successful cement pressurization with total hip arthroplasty depends on the capacity of the cement gun and its ability to pressurize the canal and the integrity of the intramedullary plug and the proximal seal used to contain the cement bolus during pressurization. In the laboratory, the authors measured the volume of cement delivered by two cement guns (from Zimmer, Warsaw, IN, and Howmedica, Rutherford, NJ) in comparison with typical values for the volume of the medullary canal following standard surgical preparation. The two cement guns studied delivered 93 and 138 mL cement, respectively. In comparison, the volume of the intramedullary canal ranged from 35 to 70 mL using a standard femoral prosthesis (Precision Hip System, Howmedica). Peak pressures developed during cement injection using the cement guns were 73.6 +/- 27.1 psi for the Zimmer system and 47.3 +/- 16.9 psi for the Howmedica system. Both devices were able to sustain a minimum pressure of at least 6.5 psi through cementing when used in conjunction with a flexible pressurizing seal. The mechanical performance of five designs of intramedullary plugs was assessed by monitoring plug displacement during cement pressurization in reamed cortical specimens. The performance of each device was judged by its ability to withstand cement pressures of 50 psi without displacement within the medullary canal. On the basis of this test, the probability that these plugs would exceed this criterion when used with the femur was estimated to range from 24 to 94%. Few of the commercially available plugs were able to withstand cement pressures routinely generated using standard cement delivery systems.

The effect of aging on the shape of the proximal femur.

The design of cementless femoral prostheses is based on the assumption that age and gender do not affect the shape of the proximal femur. To test this hypothesis, standard anteroposterior and lateral radiographs were prepared of 4 sets of 20 femora, obtained from young (range, 40-60 years) and elderly (range, 60-90 years) donors of both genders. The intracortical and extracortical borders of each femur were digitized electronically, and key parameters were measured to define the shape and dimensions of the medullary canal and the position of the femoral head. Systematic differences were observed between the size and shape of male and female femora. Extracortical dimensions were larger in the male femora by 14% to 19%, and endosteal dimensions by 11% to 24%. However, there were no significant differences between the canal shape of young male and young female femora in the coronal, sagittal, or transverse planes. The male femora displayed no significant differences in canal shape or endosteal width as a function of age. Profound differences were observed in the endosteal shape and diaphyseal dimensions of the young and old female femora. The older female femora had wider canals at the level of the isthmus, with a significant reduction in the canal flare index (the ratio between the canal width proximal to the lesser trochanter and at the isthmus). This study demonstrates that cementless femoral prostheses of 1 standard shape cannot provide a close fit to the endosteal contours of young and elderly women.

Fixation of the Chevron osteotomy utilizing absorbable polydioxanon pins.

The Chevron osteotomy has become popular for the treatment of hallux valgus deformity. Displacement of the osteotomy is a known complication. Many methods of internal fixation have previously been reported to prevent the displacement and each has inherent advantages and disadvantages. An alternative method of fixation, utilizing absorbable polydioxanon pins, is presented in this article. Between April 1989 and April 1990, 71 procedures were performed on 50 patients. At 1-year follow-up, there were no complications or infections. Review of the osteotomy after surgery and at 1 year revealed no fractures or displacements, and no pins required removal. Internal fixation utilizing absorbable polydioxanon pins appears to have significant benefits when compared with metallic fixation.

Arthrodesis of the ankle with lateral plating.

A modification of internal fixation compression arthrodesis for ankle fusion is described using two 6.5-mm cancellous bone screws and a lateral T plate. Using this technique, 20 consecutive arthrodeses by one surgeon were reviewed. Solid union was attained in 19 of 20 patients (95%). Average follow-up was 18 months (range 6-59 months). Time to obtain solid arthrodeses averaged 18 weeks. In 11 patients who returned for follow-up, clinical grading using the Mazur scale score averaged 70 of 90 points. Diagnoses included posttraumatic degenerative arthritis, failed ankle arthrodesis and rheumatoid arthritis (2 each), failed ankle arthroplasty, and post-tuberculous arthritis (1 each). Complications included one malunion and one asymptomatic screw malposition. All patients attaining union were pleased with the procedure.

Arthroscopic treatment of intraarticular fibrous bands after total knee arthroplasty. A followup note.

Eighteen consecutive patients (average age, 66.4 years at arthroscopy) who had a previous total knee arthroplasty underwent arthroscopic resection of a transverse suprapatellar fibrous band, which was causing patellar snapping and jumping as it impinged on the intercondylar notch portion of the femoral component during active extension (tethered patellar syndrome). Fourteen patients were available for followup at an average of 56 months (range, 37-91 months). All patients had complete relief of the patellar symptoms. One patient had a recurrence of symptoms 10 months after arthroscopy but remained symptom free 30 months after repeat resection. All are currently symptom free and are classified as having excellent results. The average range of motion is 1 degree to 115 degrees. There were no complications attributable to the arthroscopy. The tethered patellar syndrome has been noted by various authors, and has been most commonly treated by arthrotomy. At arthroscopy, a transverse suprapatellar fibrous band was seen to catch on the intercondylar notch of the femoral component, snapping free as the knee extended past approximately 30 degrees. In this population, excellent long-term results were achieved with arthroscopic removal of the transverse suprapatellar band.

Posterior cruciate function following total knee arthroplasty. A biomechanical study.

The strain developed in the posterior cruciate ligament (PCL) of eight fresh cadaveric knees was measured before and after total knee arthroplasty using a loading technique that simulated stair ascent and descent. Each knee was instrumented with a Hall Effect strain gauge (Micro-Strain, Burlington, VT) in the PCL, a load cell in the quadriceps tendon, an electrogoniometer, and an array of linear displacement transducers to measure femoral rollback. Testing was undertaken with each knee in its normal state with the anterior cruciate cut and with a cruciate-retaining prosthesis, a cruciate-excising prosthesis, and a cruciate-substituting prosthesis. Normal PCL strain levels were produced in only 37% of the trials following implantation of the cruciate-retaining knee arthroplasties. With a cruciate-retaining prosthesis, femoral rollback decreased by an average of 36% and was associated with a 15% loss in extensor efficiency. In the procedures performed with excision of the PCL, rollback decreased by 70% and extensor efficiency by 19%. Cruciate substitution resulted in a 12% loss in rollback and an 11% decrease in extensor efficiency. The strain developed within the PCL during knee flexion was found to be extremely sensitive to the thickness of the polymeric tibial insert. In the majority of cases, it was not possible to restore normal ligament loading with flexion while simultaneously maintaining acceptable varus/valgus stability of the knee joint. Using a range of contemporary knee arthroplasties, the authors were unable to consistently reproduce normal function of the PCL.

Effect of rotation on the radiographic appearance of the femoral canal.

Radiographic templating is a key element of preoperative planning for cementless total hip arthroplasty, and it aids in the selection of an appropriate implant. Frequently, the radiographic projection of the proximal femur does not correspond to that of the femoral prosthesis on its template due to variations in patient positioning. This discrepancy is a potential source of error when predicting which femoral component will best fit within the femoral canal. To evaluate the effect of femoral rotation on the size and shape of its radiographic image, anteroposterior and lateral radiographs of 12 femora were prepared over a range of positions. Several medullary canal dimensions were measured for each projection. The changes in these dimensions were compared using the image at neutral rotation as a reference. Each femur was then implanted with an appropriately sized cementless prosthesis to determine its actual rotational orientation in the canal. On the anteroposterior projection, statistically significant changes in the width of the proximal canal with femoral rotation were noted. There was no statistically significant change in distal canal dimensions with rotation. On the lateral projection, the dimensions of the proximal canal changed significantly with internal rotation; however, external rotation had no effect on canal dimensions. In general, the magnitude and direction of the canal dimensions were highly variable. The final rotational orientation of the femoral component in the canal was quite variable with respect to the plane of the femoral neck. Errors in implant selection may be due to excessive reliance on preoperative templating, which can be misleading because of femoral rotation.

Amputation after failed total knee arthroplasty.

From 1983 to 1992, nine above-knee amputations were performed in eight patients for complications after total knee arthroplasty. The mean time from the initial knee replacement to amputation was 9.7 years. The average age of the four men and four women was 72 years. Eight knees had chronic infection and one had intractable pain after four revision attempts for aseptic loosening. Most patients had two revision arthroplasties after the original implant. In this series, the common factors that lead to amputation were multiple revision attempts in the presence of chronic infection, severe bone loss, and intractable pain. Earlier consideration of arthrodesis rather than multiple revision attempts is recommended to avoid such a poor outcome.

Continuous epidural analgesia using fentanyl and bupivacaine after total knee arthroplasty.

Eighty consecutive patients undergoing unilateral total knee arthroplasty received postoperative analgesia consisting of a continuous epidural infusion of fentanyl and bupivacaine. Nineteen patients (24%) were unable to complete the three-day course of epidural infusion: two thirds for technical reasons and one third because of adverse effects. The remaining 61 patients (76%) successfully completed the 72-hour protocol. The benefits in this group included a shorter hospital stay (8.3 versus 9.5 days, p < 0.01) and earlier return of flexion (p < 0.05) as compared with the group that had the epidural catheter removed prematurely. When compared with previous experiences with continuous epidural analgesia using Duramorph and bupivacaine for the first three days after TKA in 56 patients, the current group of 80 patients had shorter average hospitalization (8.6 versus 9.7 days, p < 0.01) and earlier return of flexion (p < 0.001). Patient acceptance was excellent with either agent. Confusion and pruritus were significantly less common with fentanyl, but the incidence of hypotension was increased. Nausea was problematic in both groups. The incidence of respiratory depression was 5% for either opiate. The degree of hypoventilation and treatment required for it were less severe with fentanyl, however.

Radial head fracture. A potentially complex injury.

A prospective study was undertaken on 50 acute, consecutive fractures of the radial head. Clinical examination and stress testing were performed on each patient. A valgus stress radiograph, in conjunction with clinical stress testing, was used to assess competence of the medial elbow ligaments. Axial stability of the radius was determined by the axial stress test. Eleven patients (22%) sustained a comminuted radial head fracture; all had instability either to valgus or axial stress. Seventeen patients (34%) had a displaced vertical shear type fracture or an impacted fracture of the radial neck. All sustained some injury to the medial collateral ligament, with variable degrees of valgus elbow instability. Twenty-two (44%) of the fractures were minimally or nondisplaced. All 22 were stable. Radial head fracture morphology can be used to assess the likelihood of associated soft-tissue injury.