Cost comparison between bilateral simultaneous, staged, and unilateral total joint arthroplasty.

A hospital-based computer system was used to compare the inpatient costs of performing bilateral simultaneous sequential, staged, and unilateral total hip and knee arthroplasties. Bilateral simultaneous sequential total knee arthroplasty was 36% less costly than 2 unilateral total knee arthroplasties. Bilateral simultaneous sequential total hip arthroplasty saved 25% over the costs of performing 2 unilateral hip arthroplasties. Prosthetic costs range between 28% and 43% of the total costs of hospitalization. There was a significant correlation between hospital length of stay, morbidity, and total costs, but no correlation with patient age and sex except in the unilateral knee patients. Bilateral simultaneous sequential joint arthroplasty can save more than $10,000 for each total knee patient and more than $8,000 for each total hip patient.

Mechanical model for critical strain in mineralizing biological tissues: application to bone formation in biomaterials.

A simple theoretical model for the role of strain energy density in the initial mineralization of soft tissues is presented and used to derive a limit of the allowable strain in tissue engineered biomaterials. The model incorporates the mechanical energy in calcified tissue due to time-varying loads into the more commonly used energetic arguments for mineralization. By using the Voight (equal-strain) and Reuss (equal-stress) composite material models to relate the volumetric density of calcified tissue to overall material modules, two models were developed to assess the effect of an imposed overall material strain on mineralization. A rate equation based on strain energy was used to model the kinetics of mineralization, and the stability of the rate equation was assessed, leading to a limit on overall material strain based on the specific energy for mineralization of soft tissues. The result depended on the stiffness of the material in series with the mineralizing tissue. Taking the stiffness of the material in series with the tissue as infinite lead to a prediction of critical strain for mineralization in the calcifying biological tissue which was the same on the Reuss and Voight models. The interaction of this theoretical model with biological factors and some clinical implications of the model are discussed.

Comparison of general and epidural anesthesia in patients undergoing primary unilateral THR.

One hundred ninety-five consecutive patients underwent 195 primary unilateral total hip arthroplasties between January 1988 and December 1993. Patients were divided into three groups based on the type of anesthesia utilized for their procedure. Group I consisted of 108 patients (59 women and 49 men; average age 56 years) who had general endotracheal anesthesia alone. Group II consisted of 70 patients (41 women and 29 men, average age 58 years) who had general endotracheal anesthesia with epidural augmentation intraoperatively and postoperatively. Group III consisted of 17 patients (6 women and 11 men, average age 62 years) who had epidural anesthesia only. Data were analyzed by anesthesia group to compare a variety of clinically relevant factors. No statistically significant differences among groups were noted regarding average age at surgery, the underlying diagnoses leading to joint replacement, the number of preexisting medical conditions, length of hospitalization, nonsurgical operating room time, intraoperative blood transfusions, intraoperative femur fractures, deep venous thrombosis, deep infections, death, or the prevalence of postoperative urinary tract infections. Postoperative urinary tract infections correlated with duration of Foley catheterization, but not the duration of epidural catheterization. Significant differences among anesthesia groups were observed for two factors: 1) estimated intraoperative blood loss was highest for Group I (P < .05) and was primarily a function of surgical time (P < .0001), and 2) postoperative Hemovac output (over the first and second postoperative 24-hour periods) was greatest for Group II (P < .05). Epidural anesthesia appears to be a safe modality in patients undergoing primary unilateral total hip replacement.

Inpatient cost of primary total joint arthroplasty.

The financial and medical records of 98 patients undergoing either primary total hip or total knee arthroplasty were analyzed to determine the inpatient cost of their surgery. Eleven patients were excluded, leaving a total of 87 patients available for analysis who underwent 50 primary total hip and 37 primary total knee arthroplasties. Ten separate cost centers such as implant, anesthesia/operating room, nursing/hospital room, physical therapy, and laboratory were included in the evaluation. Correlations between total cost and demographic parameters such as patient sex, age, and length of stay were determined. Seventy-six percent of the inpatient cost of all total joint procedures consisted of implant, anesthesia/operating room, and nursing/hospital room costs. Reductions in the cost of these components represent a potential for cost containment. The strongest correlate for total cost was hospital length of stay (P < .0005). Unfortunately, there is evidence that length of stay has reached a plateau and cannot be reduced further without affecting the quality of care and the incidence of complications.

Bone remodelling adjacent to intramedullary stems: an optimal structures approach.

The internal parameters in bone remodelling theories often are not clearly related to the bony structure which results from the simulations in which they are implemented. For a restricted class of bone remodelling theories, we have previously found a connection between overall structural optimization and the parameters within a continuum-level remodelling rule. In this study, we assess whether a simplified analytical formula based on structural optimization can predict the behaviour of a large-scale finite element bone remodelling simulation. The analytical formula predicts when bone will remain around an intramedullary implant. The predictions of the formula are borne out in the numerical results. This leads to a physical interpretation of one of the two parameters in the remodelling rule used. The results also show some characteristics which are clinically relevant. This study extends earlier results due to Huiskes for internal remodelling around intramedullary implants by using a different, numerically stable remodelling algorithm based on optimization. The study also shows a direct practical application of the optimizing remodelling theory the authors have developed previously.

Effect of porous coating and loading conditions on total hip femoral stem stability.

An examination of femoral bone-prosthesis interface behavior under different load types is undertaken using finite-element analysis. Three-dimensional finite-element models are made of two designs of hip prostheses after implantation in a femur. Femoral geometry was determined by computed tomography scans. The models were loaded in one-legged stance and stairclimbing configurations. The implants were modeled as both smooth surfaced and porous coated. The amount of contact and the relative motion between bone and implant were calculated. It is shown that torsional loads such as occur during stairclimbing contribute to larger amounts of implant micromotion than does stance loading. Contact at the bone-prosthesis interface is more dependent on load type than on implant geometry or surface coating type.

Prosthetic component failures in hip arthroplasty surgery.

A survey was conducted of the entire membership of the American Association of Hip and Knee Surgeons to determine their experience with total hip arthroplasty (THA) device-related failures. Forty-seven percent of the membership reviewed their preceding 5-year clinical experience and returned the survey instrument. The aggregate 5-year volume encompassed experience with more than 60,000 hip arthroplasties. The median 5-year experience per respondent was 200 metal-based sockets and 214 metal stems. The frequency distribution of the number of hip arthroplasties per respondent was skewed to the right, with fewer surgeons reporting the higher volumes of arthroplasties. In aggregate, 60,115 of the acetabular components used were metal backed. Nonmetal acetabular component usage totaled 3,219. Complete polyethylene failure was defined as fracture or complete wear through of the polyethylene portion of the component. Complete polyethylene failure was seen in 172 metal-backed sockets (29/10,000). Seventy-seven all-polyethylene sockets had complete polyethylene failure (239/10,100). A total of 87 THAs were revised for modular acetabular dissociations for a dissociation rate of 15/10,000. In aggregate, 64,483 metal-stemmed components were used. Femoral stem fractures occurred in 172 for a rate of 27/10,000. A total of 56,965 metallic femoral components were reported as being modular. Dissociation between the femoral head and neck was uncommonly seen (3/10,000). Use of ceramic femoral heads was low (5,023); however, a total of 11 ceramic head fractures were revised in the 5-year period for a failure rate of 22/10,000. The risks of catastrophic prosthetic THA failure are low but vary according to hip component and the manufacturer of the part.

The effect of femoral component position on the kinematics of total knee arthroplasty.

In a laboratory study using seven fresh-frozen anatomic specimen knees, the effect of total knee arthroplasty on the three-dimensional kinematics of the patella, femur, and tibia were measured. Experiments were performed in the intact knee, after division of the anterior cruciate ligament (ACL), after total knee arthroplasty, and after 10 degrees internal rotation, 10 degrees external rotation, 5-mm medial shift, and 5-mm lateral shift of the femoral component on the femur. The presence of a high lateral ridge on the anterior surface of the femoral component effectively prevented patellar subluxation or dislocation, but displaced and tilted the patella medially. Internal rotation or medial displacement of the femoral component exaggerated this medial patellar displacement and shift. External rotation of the femoral component corrected it, except at flexion angles greater than 100 degrees, where the femur was shifted medially on the tibia and externally rotated 15 degrees. This combination produced a net 10-mm medial displacement of the patella relative to the tibia at 120 degrees knee flexion. Lateral placement of the femoral component compensated for the effect of the high lateral ridge and allowed more normal patellar tracking while allowing tibiofemoral motions similar to those seen after sectioning of the ACL. The kinematics of the patellofemoral and tibiofemoral joints were not reproduced with a total knee prosthesis that sacrifices the ACL. When using a prosthesis with a high lateral ridge, lateral placement of a femoral component prevented patellar dislocation and allowed patellar tracking patterns similar to those seen in the intact knee without further altering tibiofemoral motions.

A knowledge-based computer-aided design and manufacturing system for total hip replacement.

A knowledge-based computer-aided design and manufacturing system (CAD-CAM) has been developed for total hip replacement. Knowledge-based refers to the fact that the design process is a computer program that has been provided with preprogrammed design rules. Compared with conventional CAD-CAM systems, the knowledge-based system is automated, requires less designer intervention, and increases the accuracy of the design process. The capabilities of the system make it ideal for the design of standard and custom total hip replacement. A full-fill, press-fit custom total hip replacement has been designed using the knowledge-based system. The early clinical results of a series of 37 replacements in 31 patients is described in this paper.

Effect of patella thickness on patella strain following total knee arthroplasty.

Patellar complications following total knee arthroplasty (TKA) have begun to emerge as a major cause of failure. In an effort to understand some of the mechanical factors that might contribute to patellar component failure, a biomechanical study was performed. Quadriceps force and anterior patellar strain were measured during dynamic flexion in 10 fresh, paired human cadaver knee joints. First, tests were performed in the intact knee, followed by either posterior cruciate ligament (PCL) retention or sacrifice of TKA without patellar resurfacing. Tests were then performed following patellar resurfacing with an overly thick, optimum and thin, bony patella. Patellar strain increased in each specimen (with flexion angles of up to 80 degrees), was most pronounced as the bony patella became thinner, was closest to the intact knee when the patella was not resurfaced, and was unaffected by PCL retention or sacrifice. Patellar osteotomy, resulting in a bony patellar thickness of less than 15 mm, resulted in significantly increased strain. TKA systems should include instrumentation that allows precise restoration of overall patellar thickness while maintaining a bony patellar thickness of at least 15 mm.

Posterior fixation of thoracic and lumbar spine fractures using DC plates and pedicle screws.

A prospective investigation of internal fixation of acute thoracic and lumbar spine fractures using pedicle screws and dynamic compression plates was performed. This article details the results of 23 patients who were followed for an average of 20 months. The indications for the procedure were an unstable fracture--dislocation below the eighth thoracic vertebra or low lumbar fractures that would be difficult to treat with conventional spinal implants. The patients were divided into three groups based on the fracture pattern and the level of the fracture. Group I consisted of fracture--dislocation injuries at the thoracic level and thoracolumbar junction. Group II patients sustained lumbar fracture--dislocations, and Group III contained lumbar burst fractures. Sagittal and coronal angles as well as anterior and posterior vertebral body height were measured at the injury level preoperatively, postoperatively, and at 3-month intervals. Statistical analysis of these angles demonstrated that Group I and III patients had significant correction of their relative kyphotic deformity after surgery; however, there were no differences in kyphotic angle at the 3-month interval. Despite this eventual return to preoperative angles, all patients except one developed solid fusion. There was no increase in neurologic deficit. One patient developed an asymptomatic pseudarthrosis demonstrated by breakage of both plates through unfilled screw holes at the motion segment. Other complications included one wound infection, one case of arachnoiditis after an intradural bone fragment was excised, and one dural tear created by a Kirschner wire, which did not result in a neurologic deficit or a cerebrospinal fluid leak.(ABSTRACT TRUNCATED AT 250 WORDS)

Rehabilitation of the fracture patient.

With any type of fracture it is important that rehabilitation begin when the fracture is stable, whether the stability occurred naturally or is achieved by internal fixation.

Relation between knee motion and ligament length patterns.

Fifteen knee specimens were tested in a 6°-of-freedom test machine using quadriceps force to drive continuous flexion-extension motion. Ten of the knees were again tested following isolated transaction of the anterior cruciate ligament. From transducer outputs, three-dimensional motion was determined. Using biplanar radiography and bone sectioning, ligament insertion coordinates and joint surface geometry were determined and used to calculate ligament length ratios as a function of knee flexion. Consistent motion patterns were seen in all knees. The prominent aspects of motion were a 15° internal rotation and an 8.6 mm anterior displacement of the tibia with flexion from 0 to 120°. Anterior cruciate transaction resulted in abnormal and excessive anterior-posterior displacement in early flexion, but a return to normal in late flexion. The results indicated that under our test conditions the boundaries of anterior-posterior motion were determined primarily by the cruciate ligaments, but tibial rotation was not guided by the cruciate ligaments.

The effect of femoral component position on patellar tracking after total knee arthroplasty.

In a laboratory study, seven fresh anatomic knee specimens were evaluated to define the three-dimensional motions of the patella before and after total knee arthroplasty (TKA) with the AMK knee. The patella was displaced medially by an average of 4 mm and tilted medially by an average of 4 degrees after standard TKA. Medial translation or internal rotation of the femoral component further displaced and tilted the patella medially, but lateral translation or external rotation of the femoral component produced less predictable changes in patellar tracking. The patterns of patellar tracking after external rotation of the femoral component came closer to reproducing those of the intact knee than any other femoral component position. The high lateral ridge on the femoral component effectively prevents patellar dislocation but may produce abnormally high stresses on the patellar implant, especially if the implant is medially displaced or internally rotated. This could lead to accelerated wear or loosening of the patellar component.

Three-dimensional dynamic motion analysis of the anterior cruciate ligament deficient knee joint.

Dynamic three-dimensional motion analyses of 15 fresh human knee joints subjected to combinations of flexion velocity and moment, internal and external femoral torque, and horizontal shear before and after sectioning the ACL were performed. ACL deficient specimens demonstrated marked anterior instability without rotational instability. The pivot shift phenomenon occurred with an isolated ACL deficiency and was the result of anterior instability. The pivot shift was accentuated by external femoral torque, decreased by internal femoral torque, and was present in the absence of any applied torque. The pivot shift produced a sudden directional change in the motion of both femoral condyles and may be responsible for the meniscal degeneration that accompanies chronic ACL deficiency.

In vivo effects of axial loading on double-curve scoliotic spines.

The in vivo three-dimensional orientation of the vertebral columns in five adolescent idiopathic scoliotic subjects with double curves were examined in standing, supine, and supine positions with Cotrel traction postures using a biplanar radiographic technique. This analysis was compared with that obtained from a previous study on five healthy adolescent subjects to assess the postural changes and mechanical factors relating to the development of double-curve scoliotic deformity. Statistical analysis of vertebral orientation, rotation, disc distraction, and wedging between the normal and scoliotic subjects indicated that central intervertebral disc height and anteroposterior wedging were most consistently altered in double-curve scoliotic deformity. Since these two parameters relate directly to the geometry of the disc, the etiology of double-curve scoliosis may be related to a primary defect affecting the geometry of the intervertebral disc that results in a vertebral column which then undergoes rotational deformity.

In vivo effects of axial loading on healthy, adolescent spines.

Five healthy adolescents were studied using a three-dimensional radiographic technique to analyze the in vivo effects of standing, supine and supine Cotrel traction positioning on the orientation of spinal segments. In general the configuration of these spines was found to be relatively unaffected by changes in subject position and column loading. The major effects of supine positioning appeared to be lessening of standing lumbar lordosis while supine traction tended to diminish this effect. Measurement of rotation relative to a common reference body, L4, was influenced by the degree of AP and ML wedging throughout the column. The standing curve was found to be slightly rotated by a small but constant amount at each level and tended to derotate as a whole with supine and supine traction positioning. Measurement of successive intervertebral rotation showed effects independent of wedging with each vertebral body randomly rotated and unaffected by subject orientation. With the exception of central vertebral disk heights, all parameters measured possessed relatively wide limits of variability which appeared to be the result of true intersubject scatter as opposed to measurement error.