Sex differences in quadriceps strength in OA

The purposes of this study were to determine 1) whether sex differences in quadriceps torque and isotonic power persist when controlling for muscle volume (i. e., torque/muscle volume and power/muscle volume) in participants with knee osteoarthritis (OA) and 2) the factors responsible for potential sex differences. Isometric torque, isotonic power (the product of torque and velocity, measured at 10, 20, 30, 40 and 50% maximal voluntary contraction; MVC) and maximal unloaded velocity were assessed in men (n=16, mean age=62.1 ± 7.2) and women (n=17, mean age=60.4 ± 4.3) with knee OA. Torque and power were normalized to muscle volume. The interpolated twitch technique was used to measure voluntary activation (VA) and evoked twitch and torque-frequency characteristics were measured to obtain information about muscle fibre distribution. Torque and power at all loads were significantly lower in women (p<0.05). Sex differences in power were reduced by 50% when controlling for muscle volume but were still significant at 10-40% MVC (p<0.05). No differences in VA, torque-frequency properties or time-to-peak tension of the evoked twitch were observed (p>0.05). These results suggest that only minor sex differences in torque and power persist when controlling for muscle volume. As VA and contractile property differences were not observed, other factors seem to be responsible.

Quadriceps neuromuscular function and self-reported functional ability in knee osteoarthritis

The purposes of this study were to determine 1) the relationships of self-reported function scores in patients with knee osteoarthritis (OA) to both maximal isometric torque and to isotonic power at a variety of loads, and 2) the degree to which muscle volume (MV) or voluntary activation (VA) are associated with torque and power measures in this population. Isometric maximal voluntary contraction (MVC) torque and isotonic power [performed at loads corresponding to 10, 20, 30, 40, and 50% MVC, and a minimal load ("Zero Load")] were measured in 40 participants with knee OA. Functional ability was measured with the Western Ontario and McMaster Osteoarthritis Index (WOMAC) function subscale. MV was determined with magnetic resonance imaging, and VA was measured with the interpolated twitch technique. In general, power measured at lower loads (Zero Load and 10-30% MVC, r(2) = 0.21-0.28, P < 0.05) predicted a greater proportion of the variance in function than MVC torque (r(2) = 0.18, P < 0.05), with power measured at Zero Load showing the strongest association (r(2) = 0. 28, P < 0.05). MV was the strongest predictor of MVC torque and power measures in multiple regression models (r(2) = 0.42-0.72). VA explained only 6% of the variance in MVC torque and was not significantly associated with power at any load (P > 0.05). Quadriceps MVC torque and power are associated with self-reported function in knee OA, but muscle power at lower loads is more predictive of function than MVC torque. The variance in MVC torque and power between participants is due predominantly to differences in MV and has little to do with deficits in VA.

Effect of polyethylene component thickness on range of motion and stability in primary total knee arthroplasty

Total knee arthroplasty (TKA) is a common procedure with good survivorship and functional results. Optimal results are dependent on proper osseous cuts and soft tissue balancing. Soft tissue tensioning via the polyethylene spacer thickness is an important component of soft tissue balancing. Increased thickness increases soft tissue tension and, therefore, has the potential to increase stability but decrease range of motion (ROM). Decreased polyethylene thickness may decrease soft tissue tension and has the potential to increase ROM but decrease stability. Using computer-based navigation, the intraoperative effect of increasing and decreasing polyethylene thickness in 1-mm increments on ROM and coronal stability throughout the ROM of 35 patients was examined. It was found that increasing the polyethylene thickness by 1-mm increments had a statistically significant impact on the ability to achieve full extension but had no impact on flexion. Increased polyethylene thickness decreased coronal plane motion. Coronal plane laxity increased with increased flexion irrespective of polyethylene thickness. In this patient cohort, lateral laxity became >1° when the knee was flexed. However, medial structures prevented valgus angulation of >1° in all scenarios except when the polyethylene was diminished by 2 mm. Changes in polyethylene thickness had an impact on the ability to gain full extension and coronal plane motion.

Vastus medialis motor unit properties in knee osteoarthritis

BACKGROUND

Maximal isometric quadriceps strength deficits have been widely reported in studies of knee osteoarthritis (OA), however little is known about the effect of osteoarthritis knee pain on submaximal quadriceps neuromuscular function. The purpose of this study was to measure vastus medialis motor unit (MU) properties in participants with knee OA, during submaximal isometric contractions.

METHODS

Vastus medialis motor unit potential (MUP) parameters were assessed in 8 patients with knee OA and 8 healthy, sex and age-matched controls during submaximal isometric contractions (20% of maximum isometric torque). Unpaired t-tests were used to compare groups for demographic and muscle parameters.

RESULTS

Maximum knee extension torque was ~22% lower in the OA group, a difference that was not statistically significantly (p = 0.11). During submaximal contractions, size related parameters of the needle MUPs (e.g. negative peak duration and amplitude-to-area ratio) were greater in the OA group (p < 0.05), with a rightward shift in the frequency distribution of surface MUP negative peak amplitude. MUP firing rates were significantly lower in the OA group (p < 0.05).

CONCLUSIONS

Changes in MU recruitment and rate coding strategies in OA may reflect a chronic reinnervation process or a compensatory strategy in the presence of chronic knee pain associated with OA.

Wear rate of highly cross-linked polyethylene in total hip arthroplasty. A randomized controlled trial

BACKGROUND

Highly cross-linked polyethylene was introduced for clinical use in total hip arthroplasty with the expectation that it would exhibit less wear when compared with conventional polyethylene. The purpose of this study was to report the clinical and radiographic results, after a minimum of five years of follow-up, of a randomized, blinded, controlled trial comparing a conventional polyethylene with a first-generation highly cross-linked polyethylene.

METHODS

One hundred patients were enrolled in a prospective, randomized controlled study comparing highly cross-linked and conventional polyethylene acetabular liners in total hip arthroplasty. Fifty patients were in each group. At the time of follow-up, clinical outcomes were assessed and steady-state femoral head penetration rates (after bedding-in) for each patient were calculated with use of a validated radiographic technique. In addition, a statistical comparison of polyethylene wear between groups was performed with use of generalized estimating equations.

RESULTS

At a mean of 6.8 years postoperatively, there were no differences between the two polyethylene groups with regard to the Harris hip score, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), or Short Form-12 (SF-12) score. The mean femoral head penetration rate in the first through fifth years was found to be significantly lower in the group treated with the highly cross-linked polyethylene (0.003 mm/yr [95% confidence interval, +/-0.027]) than it was in the group treated with conventional polyethylene (0.051 mm/yr [95% confidence interval, +/-0.022]) (p=0.006). Men treated with a conventional polyethylene liner had a significantly higher (p<or=0.012) femoral head penetration rate (0.081 mm/yr [95% confidence interval, +/-0.065]) than both men and women with a highly cross-linked liner (-0.013 mm/yr [95% confidence interval, +/-0.074] and 0.009 mm/yr [95% confidence interval, +/-0.028], respectively). The general estimating equations demonstrated that the group with a highly cross-linked polyethylene liner had a significantly lower femoral head penetration rate than the group with a conventional polyethylene liner (p=0.025), and a significantly higher femoral head penetration rate was demonstrated in men with a conventional polyethylene liner when compared with both men and women with a highly cross-linked liner (p=0.003).

CONCLUSIONS

At a minimum of five years postoperatively, the steady-state femoral head penetration rate associated with this first-generation highly cross-linked polyethylene liner was significantly lower than that associated with a conventional polyethylene liner. Long-term follow-up is required to demonstrate the clinical benefit of this new material.

Load balance in total knee arthroplasty: an in vitro analysis

BACKGROUND

One of the goals of total knee arthroplasty (TKA) is to balance the loads between the compartments of the knee. An instrumented load cell that measures compartment loads in real time is utilized to evaluate conventional, qualitative methods of achieving this balance.

METHODS

TKA was performed on 10 cadaveric knees. Prior to and after load balancing, compartment forces were measured at flexion angles of 0-90 degrees. Knees were randomly assigned into one of two groups, based upon whether or not the surgeons could visualize the load cell's output during balancing.

RESULTS

Prior to attempting load balance, there were significant differences between the medial and lateral compartment loads for all knees (p < 0.05). After attempting balance with the aid of the load cell, there was equal load balance at all angles studied. Without the aid of the load cell, balance was not consistently achieved at every angle.

CONCLUSIONS

Conventional load balancing techniques in TKA are not perfect.

The influence of implant articular thickness and glenohumeral conformity on stability of an all-metal glenoid component

The objective of this study was to determine the effect of implant thickness and glenohumeral conformity on fixation of an all-metal glenoid component. A stainless steel glenoid component was designed and implanted in 10 cadaveric scapulae. A testing apparatus capable of producing a loading vector at various angles, magnitudes, and directions was used. The independent variables included 6 directions and 3 angles of joint load, 3 implant thicknesses, and 4 glenohumeral conformities. Implant micromotion relative to bone was measured by use of 4 displacement transducers at the superior, inferior, anterior, and posterior sites. The components displayed a consistent response to loading of ipsilateral compression and contralateral distraction. Stability decreased as the load application angle increased (P < .05). A decrease in the implant thickness and glenohumeral conformity resulted in increased implant stability (P < .05). Decreasing implant thickness and glenohumeral conformity reduce the eccentric component of loading and may improve the durability of glenoid implants.

Computer-assisted gap equalization in total knee arthroplasty

This in vitro biomechanical study compared a conventional balancing technique in knee arthroplasty to a technique using computer assistance. The experimental technique used a soft tissue tensioner instrumented with computer-monitored load cells to quantify soft tissue tension. To assess outcome, a tibial load transducer measured medial and lateral compartment forces and tibial rotation. An electromagnetic tracking system quantified knee position under simulated muscle loading. The computer-assisted technique improved knee balance before insertion of components. However, once components were implanted, there was no difference in knee load balance between the 2 techniques. No correlation was shown between compartmental load balance and tibial rotation or mechanical axis misalignment. Although computer-assisted surgery may improve technical accuracy, further work is necessary to achieve an optimal final knee load balance.

Glenoid vault endosteal dimensions: an anthropometric study with special interest in implant design

An understanding of the morphology of the glenoid is important from the viewpoint of implant design and selection. This study describes the endosteal dimensions and shape of the glenoid and correlates these results with age, gender, and the presence of osteoarthritis. This study used 72 scapulae. Data were obtained from computed tomography scans of both cadaveric and in vivo glenoids. The glenoid is relatively straight-sided in the coronal plane and more highly fluted in the transverse plane. The endosteal dimensions were larger for male specimens, but there was no difference in endosteal shape with respect to gender. These findings were not influenced by age or the presence of osteoarthritis. This study suggests that traditional glenoid component designs may not be optimal. To maximize fixation, a rectangular keel may be most effective in the coronal plane and a triangular keel may be most effective in the transverse plane.

Patellar position after total knee arthroplasty: influence of femoral component malposition

Patellar shift, tilt, and rotation were analyzed in 7 cadaveric knee specimens during simulated quadriceps loading, in the intact knee, and after implant reconstruction. Femoral component medialization, lateralization, and external rotation were also investigated. Relative motion of the patella with respect to the femur was measured using an electromagnetic tracking system. The spatial position of the patella did not change with standardized total knee arthroplasty (P <.05). After malpositioning of the femoral component, patellar rotation also did not change (P >.05); however, patellar tilt was altered by femoral component external rotation malposition (P <.05), and patellar shift was affected by all femoral component malpositions (P <.05). The spatial position of the patella relative to the femoral shaft was changed with any femoral component malposition, suggesting that the soft tissues were abnormally tensioned. This could result in subsequent wear on the patellar component and, therefore, early failure.

Does keel size, the use of screws, and the use of bone cement affect fixation of a metal glenoid implant?

The objective of this study was to determine the effect of screws and keel size on the fixation of an all-metal glenoid component. A prototype stainless-steel glenoid component was designed and implanted in 10 cadaveric scapulae. A testing apparatus capable of producing a loading vector at various angles, magnitudes, and directions was used. The independent variables included six directions and three angles of joint load, and five fixation modalities-three different-sized cross-keels (small, medium, and large), screws, and bone cement. Implant micromotion relative to bone was measured by four displacement transducers at the superior, inferior, anterior, and posterior sites. The components displayed a consistent response to loading of ipsilateral compression and contralateral distraction. Use of progressively larger keels did not significantly improve implant stability. Stability decreased as the angle of load application increased (P <.05). Screw and cement fixation resulted in the most stable fixation (P <.05).

Metal-on-metal versus polyethylene in hip arthroplasty: a randomized clinical trial

A prospective, randomized, blinded clinical trial was done to evaluate polyethylene versus metal bearing surfaces in total hip replacement. Forty-one patients were randomized to receive either a metal (23 patients) or a polyethylene (18 patients) insert. The femoral and acetabular components were identical with the acetabular insert the only variable. Patients were assessed preoperatively and postoperatively using radiographs, multiple outcome measures (Western Ontario MacMaster University Score, Harris hip score, Short Form-12), erythrocyte metal ion analysis (cobalt, chromium, titanium), and urine metal ion analysis (cobalt, chromium, titanium). Patients were followed up for a minimum of 2 years (mean 3.2 years; range, 2.2-3.9 years). There were no differences in radiographic outcomes or outcome measurement tools between patients. Patients receiving a metal-on-metal articulation had significantly elevated erythrocyte and urine metal ions compared with patients receiving a polyethylene insert. Patients who had metal-on-metal inserts had on average a 7.9-fold increase in erythrocyte cobalt, a 2.3-fold increase in erythrocyte chromium, a 1.7-fold increase in erythrocyte titanium, a 35.1-fold increase in urine cobalt, a 17.4-fold increase in urine chromium, and a 2.6-fold increase in urine titanium at 2 years followup. Patients receiving a polyethylene insert had no change in erythrocyte titanium, urine cobalt, or urine chromium and a 1.5-fold increase in erythrocyte cobalt, a 2.2-fold increase in erythrocyte chromium, and a 4.2-fold increase in urine titanium. Forty-one percent of patients receiving metal-on-metal articulations had increasing metal ion levels at the latest followup.

Design and validation of an unconstrained loading system to measure the envelope of motion in the rabbit knee joint

An unconstrained loading system was developed to measure the passive envelope of joint motion in an animal model commonly used to study ligament healing and joint arthritis. The design of the five-degree-of-freedom system allowed for unconstrained knee joint loading throughout flexion with repeated removal and reapplication of the device to a specimen. Seven New Zealand White rabbit knees were subjected to varus, valgus, internal and external loads, and the resulting envelopes of motion were recorded using an electromagnetic tracking device. Intra-specimen reproducibility was excellent when measured in one specimen, with maximal rotational differences of 0.6 and 0.3 deg between the fourth and fifth testing cycles for the varus (VR) and valgus (VL) envelopes, respectively. Similarly, the maximal internal (INT) and external (EXT) envelope differences were 0.5 and 0.4 deg, respectively, between the fourth and fifth cycles. Good inter-animal envelope reproducibility was also observed with consistent motion pathways for each loading condition. A maximal VR-VL laxity of 17.9 +/- 2.3 deg was recorded at 95 deg flexion for the seven knees tested. The maximal INT-EXT laxity of 75.2 +/- 4.8 deg occurred at 50 deg flexion. Studies on measurement reproducibility of re-applying individual testing components demonstrated a maximal error of 1.2 +/- 0.7 deg. Serial removal and re-application (test-retest) of the complete measuring system to one cadaveric knee demonstrated maximal envelope differences of less than 0.7 deg for VR-VL rotation and 2.1 deg for INT-EXT rotation. Our results demonstrate that the measuring system is reproducible and capable of accurate evaluation of knee joint motion. Baseline in vitro data were generated on normal joint kinematics for future in-vivo studies with this system, evaluating ligament healing and disease progression in arthritis models.

A biomechanical analysis of intrascaphoid compression using the 3.00 mm Synthes cannulated screw and threaded washer: an in vitro cadaveric study

This study evaluated the compressive capabilities of the 3.0 mm Synthes cannulated screw and threaded washer. A transverse osteotomy was performed at the waist of eight cadaveric scaphoids and a custom-designed load cell was inserted before internal fixion with a 3.0 mm cannulated screw and threaded washer. The mean intrascaphoid compression achieved was 108 (SD, 60) N. This compressive force is comparable to that produced by standard cortical screws.

Effect of screw placement on fixation in the humeral head

The objectives of this study were (1) to determine the most advantageous screw locations within the humeral head when plate and screw fixation is to be used and (2) to determine the effect of positioning the screw tip abutting the subchondral bone. Ten paired humeral heads were harvested with a monoplanar cut through the anatomic neck. Through use of a standardized template, 7 holes were drilled and tapped in each specimen for insertion of 6.5-mm fully threaded cancellous screws perpendicular to the plane of the cut. Paired specimens were randomized into 2 groups, one with the screw purchase in central cancellous bone and the other with the screw purchase up to the subchondral bone. Each screw was pulled out axially at a displacement rate of 10 mm/min through use of a servohydraulic testing machine. The length of thread purchase, position within the head, and screw pullout load to failure were recorded. The normalized pullout force to failure was calculated by dividing absolute pullout force to failure by length of screw purchase. Data were analyzed by means of a 2-way repeated measures analysis of variance and post hoc Student-Newman-Keuls test. The central position had a significantly higher absolute pullout force to failure than all other sites (P < .05). By virtue of the humeral head shape, the central position also had a significantly greater length of screw purchase than all other positions (P < .05). The central position had a significantly higher relative pullout force to failure than all other positions (P < .05). Subchondral bone abutment positioning improved both the absolute and the relative pullout forces to failure (P < .05). When screws and plates are used in open reduction and internal fixation of a proximal humerus fracture, a major mode of failure is loss of fixation within the humeral head. On the basis of this study, optimal screw purchase with respect to bone fixation can be achieved by including screws located in the center of the humeral head in the subchondral abutment position. To minimize screw fixation failure, the anterosuperior position should be avoided. The pattern of distribution of the relative pullout force as measured in this study is consistent with previous observational studies of patterns of trabecular density within the humeral head.

The effect of cement restrictors on the occlusion of the humeral canal: an in vitro comparative study of 2 devices

An in vitro study investigated the efficacy of cement restrictor devices on cement containment and penetration within the humerus. Eight pairs of preserved humeri were prepared using advanced cementing technique followed by insertion of 1 of 2 cement restrictors. Low-viscosity cement was injected followed by sham humeral stem insertion. Each specimen was sectioned into transverse 10-mm slices. Slices were photographed and digitized to quantify the cement/stem and canal cross-sectional area. Cement penetration was determined from the ratio of cement area to canal area. A significant increase in cement penetration was observed among slices from distal to proximal for both restrictors (P = .02). There was no significant difference in cement penetration, leakage, or migration between restrictors. Five of the 8 specimens migrated, with means of 21.5 +/- 25.0 mm and 24.0 +/- 36.0 mm for the polyethylene and silicone restrictors. Leakage or migration resulted in a significant decrease in cement penetration with the polyethylene restrictors (P = .001). In the silicone restrictor group, migration resulted in decreased cement penetration (P = .04). When using advanced cementing techniques, intramedullary restrictors allow improved cement penetration; however, they do not ensure cement containment.

Modified continuous femoral three-in-one block for postoperative pain after total knee arthroplasty

We prospectively studied the continuous "modified" femoral three-in-one block for postoperative pain after total knee arthroplasty. Sixty-two patients undergoing elective knee arthroplasty under spinal anesthesia with bupivacaine (B) and fentanyl were randomized to receive 0.2% B, 0.1% B, or placebo at 10 mL/h for 48 h after an initial bolus of 30 mL of the same solution via the femoral block catheter. The catheters were inserted under the fascia iliaca using a "double pop" technique and a peripheral nerve stimulator and were advanced 15-20 cm cranially. Venous plasma levels of B, desbutylbupivacaine, and 4-hydroxy B were measured daily for 3 days. All patients received patient-controlled analgesia with morphine and indomethacin suppositories for 48 h. Using computed tomography, we evaluated the catheter location for 20 patients. The catheter tips, located superior to the upper third of the sacroiliac joint in the psoas sheath, were labeled as ideally located. The group receiving 0.2% B had a larger block success rate, smaller morphine consumption in the immediate postoperative period (15 vs 22 mg) and during the first postoperative day (9 vs 18 mg), and achieved a greater range of motion in the immediate postoperative period (91 degrees +/- 10 degrees vs 80 degrees + 13 degrees ). Visual analog scores for pain during both rest and activity were low but similar between the groups. Forty percent of the catheters evaluated were ideally located. Ideal location and use of 0.2% B resulted in 100% success of blockade of all three nerves. The S1 root was blocked in up to 76% of patients. The plasma levels of B, 4-hydroxy B, and desbutylbupivacaine were below the toxic range during the infusion. We conclude that continuous fascia iliaca block with 0.2% B results in opioid-sparing and improved range of motion during the immediate postoperative period. Larger doses of bupivacaine may safely be used in the immediate postoperative period if needed.

IMPLICATIONS

Continuous fascia iliaca block with 0.2% bupivacaine reduces opioid requirements and improves range of motion in the immediate postoperative period compared with a placebo and 0.1% bupivacaine. Plasma levels are below the toxic range with this dose. Only 40% of the catheters are positioned in the ideal location. With the smaller dose of bupivacaine, the success rate with this block is small.

Load relaxation and forces with activity in Hoffman external fixators: a clinical study in patients with Colles' fractures

A small-frame Hoffman external fixation bar instrumented with strain gauges to quantify bending and torsional forces was applied to 4 patients with a displaced metaphyseal fracture of the distal radius. Measurements were taken during surgery as well as at 1, 3, and 6 weeks after surgery during activities of daily living and hand therapy mobilization. Radiographs also were taken before and after reduction and at each subsequent visit. Force decay occurred after reduction of the fracture, averaging only 26% of the initial distraction forces by 5 minutes. These forces plateaued and did not significantly change over the subsequent 40-minute observation period. There was no correlation between carpal height index and the forces measured in the external fixator. Significant changes in external fixator forces were measured during activities of daily living and hand therapy mobilization, but these returned to baseline after the activities were performed. The most provocative activities studied were twisting a doorknob and lifting heavy objects. These activities should be performed with caution by patients with unstable distal radial fractures.

Effect of the posterior cruciate ligament in knee-joint proprioception in total knee arthroplasty

The primary purpose of the study was to examine the role of the posterior cruciate ligament (PCL) in knee-joint proprioception after total knee arthroplasty (TKA). Knee-joint proprioception was measured in 10 patients with nonsacrificed PCL TKAs and 10 with sacrificed PCL TKAs. Knee-joint proprioception was evaluated through reproduction of static knee angles using a Penny and Giles electrogoniometer. The primary variable was absolute angular error (AAE). AAE was defined as the absolute value of the difference between the test angle and the patient's perceived version of the test angle. Proprioception deficit was compared to the WOMAC questionnaire which evaluates pain, stiffness, and physical function of the lower extremity. No significant difference was found between the nonsacrificed PCL TKA (4.33 degrees +/- 1.52 degrees) and sacrificed PCL TKA (4.38 degrees +/- 1.39 degrees) AAE values (P > .4). Furthermore, no significant differences were observed in the WOMAC questionnaire scores for all three parameters between the two types of knee prosthesis (P > .35). The current findings suggest that the preservation of the PCL in TKA may not improve knee-joint proprioception and subsequently may not improve TKA functional performance.

Advanced cement technique improves fixation in elbow arthroplasty

An in vitro study was conducted to assess the efficacy of advanced cementing techniques in the fixation of the humeral stem in elbow arthroplasty. Sixteen fresh frozen cadaveric distal humeri were prepared to accept an acrylic sham humeral stem. Conventional cementing technique using doughy cement manually inserted and packed was performed in 8 specimens. The advanced cementing technique, consisting of canal irrigation, brushing and drying with gauze, canal plugging, and low viscosity cement pressurization with a delivery system was performed in the other specimens. All humeri subsequently were sectioned into 10 mm thick sections, photographed, and digitized to quantify the perimeter and area of the stem, cement mantle, corticocancellous junction, and cement voids. The degree of cement filling, determined from the area fraction of cement to the total available area within the corticocancellous junction, was significantly greater in the advanced group compared with the conventional group. Mechanical assessment of the specimens consisted of a push out load to failure test of the cement mantle from bone. For all locations in the distal humerus, the failure load and failure stress in the advanced group was significantly greater than the conventional group. It is concluded that development of an effective cement restrictor and application of advanced cement techniques in vivo should improve the initial fixation of the humeral component and may decrease the incidence of aseptic loosening associated with elbow arthroplasty.

The cementless anatomic medullary locking femoral component: an independent clinical and radiographic assessment

OBJECTIVE

To review the clinical performance of the anatomic medullary locking (AML) femoral stem in total hip arthroplasty.

DESIGN

A clinical and radiographic review.

SETTING

A tertiary lower limb joint replacement centre.

PATIENTS

Two hundred and twenty-one patients with noninflammatory gonarthrosis.

INTERVENTIONS

Two hundred and twenty-seven primary total hip arthroplasties with the noncemented AML component completed by two surgeons.

MAIN OUTCOME MEASURES

Independent review by two experienced reviewers of the postoperative Harris hip score, radiographs of component fixation, size and degree of diaphyseal fill.

RESULTS

Harris hip score was 84 (range from 43 to 98); component fixation showed bone ingrowth in 41%, stable fixation with fibrous ingrowth in 56% and unstable fixation in 3%; severe thigh pain in 4% of cases correlated with unstable fixation, and there was mild thigh pain in 20% of cases.

CONCLUSION

The AML femoral stem performs well in replacement arthroplasty compared with other noncemented stems.

Accuracy of an electromagnetic tracking device: a study of the optimal range and metal interference

The positional and rotational accuracy of a direct-current magnetic tracking device commonly used in biomechanical investigations was evaluated. The effect of different metals was also studied to determine the possibility of interference induced by experimental test fixtures or orthopaedic implants within the working field. Positional and rotational data were evaluated for accuracy and resolution by comparing the device output to known motions as derived from a calibrated grid board or materials testing machine. The effect of different metals was evaluated by placing cylindrical metal samples at set locations throughout the working field and comparing the device readings before and after introducing each metal sample. Positional testing revealed an optimal operational range with the transmitter and receiver separation between 22.5 and 64.0 cm. Within this range the mean positional error was found to be 1.8 percent of the step size, and resolution was determined to be 0.25 mm. The mean rotational error over a 1-20 degree range was found to be 1.6% of the rotational increment with a rotational resolution of 0.1 degrees. Of the metal alloys tested only mild steel produced significant interference, which was maximum when the sample was placed adjacent to the receiver. At this location the mild steel induced a positional difference of 5.26 cm and an angular difference of 9.75 degrees. The device was found to be insensitive to commonly used orthopaedic alloys. In this study, the electromagnetic tracking device was found to have positional and rotational errors of less than 2 percent, when utilized within its optimal operating range. This accuracy combined with its insensitivity to orthopaedic alloys should make it suitable for a variety of musculoskeletal research investigations.

Some basic biomechanical characteristics of medullary pressure generation during reaming of the femur

We measured femoral intramedullary pressures and applied axial thrust force generated in vitro during reaming with the AO and Zimmer systems. Six pairs of cadaver femora were instrumented with pressure taps midshaft and in the distal diaphysis, a load cell distally to measure force, and a displacement transducer to monitor reamer position. Following initial hand reaming, intramedullary power reaming was conducted utilizing a 9-mm reamer initially, with subsequent increases in steps of 0.5 mm. All femora were maintained at 37 degrees C and albumin was used to maintain a fluid-filled canal. The highest pressures consistently occurred during initial power reaming, with peak pressures ranging from 270 to 1500 mmHg amongst femora with the AO system. No significant differences were found in the peak pressures generated for the two systems (P = 0.10). The pressure measurements at the two locations in the femur were consistently similar, indicating that pressures are continuous throughout this aspect of the femur. The pressures were not correlated with instantaneous applied axial thrust (R2 = 0.191), and this could be attributed chiefly to the additional friction force of cutting. While pressure generation in the medullary canal upon reaming is likely governed by the rate of clearance of canal content, this is a highly variable response produced by characteristics of the femur which are still not fully understood.

Compression forces generated by the Huene alignment guide alone

To enhance the fixation of scaphoid fractures by providing precise alignment and compression at the fracture site, the Huene alignment guide has been used in conjunction with bone screws. In an effort to predict the compressive force generated by the Huene guide alone, these forces were measured with a calibrated, strain-gauge, instrumented device. Three orthopedic surgeons each tested the guide 10 times on one cadaver scaphoid bone specimen. The magnitude of the compressive forces generated ranged from 9.0 to 67.0 N (mean 36.0 +/- 15.0 N). Comparisons of the force measurements among the individuals revealed no significant differences. This wide range of measurements could be attributed to the variability of the ratchet mechanism on the device. Because the Huene alignment guide is used to generate transcaphoid compressive forces, these results demonstrate that it does not yield consistent compression.

Cubitus varus: significant factors

For better understanding of the etiology of "post-supracondylar fracture cubitus varus," an in vitro anatomic experiment was performed. Elbow models were precisely photographed in 256 combinations of 10 degrees increments of varus angulation, posterior angulation, internal rotation, and/or flexion contracture. Varus angulation was the most important single factor contributing to deformity. Addition of flexion contracture or posterior angulation to a given varus angulation decreased apparent deformity, whereas addition of internal rotation worsened the deformity. Control of varus angulation in the clinical setting, by whatever method, should minimize post-supracondylar fracture cubitus varus.

Short arm plaster cast for distal pediatric forearm fractures

Ten years' clinical experience with below-elbow plaster cast treatment of distal one third pediatric forearm fractures was subjected to an independent retrospective radiographic review. In the study population of 761 fractures, no significant displacement occurred while the forearm remained in plaster. The average angulation change was 4.5 degrees (SD +/- 2.2 degrees). In each angulation change > 5 degrees, poor cast molding was evident, as reflected by a high "cast index" (p < 0.01). Although this technique is technically demanding, excellent results are obtained in all distal pediatric forearm fractures if proper cast molding is used.

An evaluation of three loading configurations for the in vitro testing of femoral strains in total hip arthroplasty

Despite recent advances in total hip arthroplasty, proximal femoral resorption and osteopenia remain problems. To analyze the proximal strain effects of three different loading conditions, strains produced in intact and postarthroplasty femora have been compared. Ten adult cadaveric femora of similar size, shape, and rigidity were tested. Ten strain-gauge rosettes were positioned on each femur. To simulate the "single leg support" phase of gait, fixtures were developed to load the femora under three different configuration: the VS (vertical shaft) configuration, with a vertically orientated femur having rotational freedom proximally and distally; the ITB (iliotibial band) configuration of a femur with rotational freedom positioned 11 degrees from the vertical, with a strain-gauge adjustable metallic simulation of the lateral muscles of the thigh; and the ABD (abductor) configuration of a femur with rotational freedom positioned 11 degrees from the vertical, with a strain-gauge adjustable metallic simulation of the abductor muscles. Each femur was loaded less than or equal to 600 N through the medial point, located at one sixth of the transcondylar distance. Strain patterns and magnitudes produced by the three loading configurations were quite different in both the intact and postarthroplasty femora. Both the ITB and the ABD configurations resulted in greater proximal medial compression and lateral tension than did the VS configuration. The magnitudes of the proximal strains were significantly greater in the ABD configuration (p less than 0.05). Postarthroplasty femora showed similar proximal results. It is proposed that meaningful strain data for the physiologically loaded femur can be obtained only with simulations that include the forces produced by the iliotibial band. To overcome the indeterminate nature and biological variation in these forces, the studies have to consider a range of forces.