Scaphoid tuberosity excursion is minimized during a dart-throwing motion: A biomechanical study.

PURPOSE: The purpose of this study was to determine whether the excursion of the scaphoid tuberosity and therefore scaphoid motion is minimized during a dart-throwing motion. METHODS: Scaphoid tuberosity excursion was studied as an indicator of scaphoid motion in 29 cadaver wrists as they were moved through wrist flexion-extension, radioulnar deviation, and a dart-throwing motion. RESULTS: Study results demonstrate that excursion was significantly less during the dart-throwing motion than during either wrist flexion-extension or radioulnar deviation. CONCLUSION: If the goal of early wrist motion after carpal ligament or distal radius injury and reconstruction is to minimize loading of the healing structures, a wrist motion in which scaphoid motion is minimal should reduce length changes in associated ligamentous structures. Therefore, during rehabilitation, if a patient uses a dart-throwing motion that minimizes his or her scaphoid tuberosity excursion, there should be minimal changes in ligament loading while still allowing wrist motion. STUDY DESIGN: Bench research, biomechanics, and cross-sectional. LEVEL OF EVIDENCE: Not applicable. The study was laboratory based.

Force variations in the distal radius and ulna: effect of ulnar variance and forearm motion.

PURPOSE: To better define normal wrist joint forces during wrist motion and forearm motion at specific wrist and forearm positions and to see if there is a relationship between these forces and the amount of ulnar variance. A secondary purpose was to determine the relationship between the thickness of the articular disk of the triangular fibrocartilage complex and the amount of force transmitted through the distal ulna. METHODS: Multi-axis load cells were attached to the distal radius and ulna of 9 fresh cadaver forearms. The axial radial and ulnar compressive forces were recorded while each wrist was moved through wrist and forearm motions using a modified wrist joint simulator. During each motion, the tendon forces required to cause each motion were recorded. The ulnar variance and triangular fibrocartilage complex articular disc thickness were measured. RESULTS: The axial force through the distal ulna and the wrist extensor forces were greatest with the forearm in pronation. No relationship was found between the amount of force through the distal ulna and the amount of ulnar variance. A strong inverse relationship was found between the triangular fibrocartilage complex thickness and the ulnar variance. CONCLUSIONS: Wrists with positive ulnar variance have generally been thought to transmit greater loads across the distal ulna, which has been felt to predispose these wrists to the development of ulnar impaction. The results of this study appear to show that all wrists have similar loading across the distal ulna regardless of ulnar variance. By comparison, pronation relatively increases loading across the distal ulna. CLINICAL RELEVANCE: Because these results suggest that within reasonable ranges of ulnar variance loading across the distal ulna is independent of ulnar variance, the clinically observed incidence of ulnar impaction is more likely the result of increased wear on a thinner and less durable triangular fibrocartilage complex than due to increased distal ulna loading in ulnar positive variant wrists.

4-corner arthrodesis and proximal row carpectomy: a biomechanical comparison of wrist motion and tendon forces.

PURPOSE: Controversy exists as to whether a proximal row carpectomy (PRC) is a better procedure than scaphoid excision with 4-corner arthrodesis for preserving motion in the painful posttraumatic arthritic wrist. The purpose of this study was to determine how the kinematics and tendon forces of the wrist are altered after PRC and 4-corner arthrodesis. METHODS: We tested 6 fresh cadaver forearms for the extremes of wrist motion and then used a wrist simulator to move them through 4 cyclic dynamic wrist motions, during which time we continuously recorded the tendon forces. We repeated the extremes of wrist motion measurements and the dynamic motions after scaphoid excision with 4-corner arthrodesis, and then again after PRC. We analyzed extremes of wrist motion and the peak tendon forces required for each dynamic motion using a repeated measures analysis of variance. RESULTS: Wrist extremes of motion significantly decreased after both the PRC and 4-corner arthrodesis compared with the intact wrist. Wrist flexion decreased on average 13° after 4-corner arthrodesis and 12° after PRC. Extension decreased 20° after 4-corner arthrodesis and 12° after PRC. Four-corner arthrodesis significantly decreased wrist ulnar deviation from the intact wrist. Four-corner arthrodesis allowed more radial deviation but less ulnar deviation than the PRC. The average peak tendon force was significantly greater after 4-corner arthrodesis than after PRC for the extensor carpi ulnaris during wrist flexion-extension, circumduction, and dart throw motions. The peak forces were significantly greater after 4-corner arthrodesis than in the intact wrist for the extensor carpi ulnaris during the dart throw motion and for the flexor carpi ulnaris during the circumduction motion. The peak extensor carpi radialis brevis force after PRC was significantly less than in the intact wrist. CONCLUSIONS: The measured wrist extremes of motion decreased after both 4-corner arthrodesis and PRC. Larger peak tendon forces were required to achieve identical wrist motions with the 4-corner arthrodesis compared with the intact wrist. We observed smaller forces for the PRC. CLINICAL RELEVANCE: These results may help explain why PRC shows early clinical improvement, yet may lead to degenerative arthritis.

Reverse shoulder arthroplasty glenoid fixation: is there a benefit in using four instead of two screws?

BACKGROUND: To allow osseous integration to occur and thus provide long-term stability, initial glenoid baseplate fixation must be sufficiently rigid. A major contributing factor to initial rigid fixation is baseplate screw fixation. Current baseplate designs use a 4-screw fixation construct. However, recent literature suggests adequate fixation can be achieved with fewer than 4 screws. The purpose of the present study was to determine whether a 4-screw construct provides more baseplate stability than a 2-screw construct. METHODS: A flat-backed glenoid baseplate with 4 screw hole options was implanted into 6 matched pairs of cadaver scapulas using standard surgical technique. Within each pair, 2 screws or 4 screws were implanted in a randomized fashion. A glenosphere was attached allowing cyclic loading in an inferior-to-superior direction and in an anterior-to-posterior direction. Baseplate motion was measured using 4 linear voltage displacement transducers evenly spaced around the glenosphere. RESULTS: There was no statistical difference in the average peak central displacements between fixation with 2 or 4 screws (P = .338). Statistical increases in average peak central displacement with increasing load (P < .001) and with repetitive loading (P < .002) were found. CONCLUSION: This study demonstrates no statistical difference in baseplate motion between 2-screw and 4-screw constructs. Therefore, using fewer screws could potentially lead to a reduction in operative time, cost, and risk, with no significant negative effect on overall implant baseplate motion.

Scaphoid excision and midcarpal arthrodesis: the effect of triquetral excision--a biomechanical study.

PURPOSE: To evaluate the biomechanical alterations that occur after traditional scaphoid excision and midcarpal arthrodesis with and without excision of the triquetrum. The hypothesis of this study was that removal of the triquetrum increases the radiolunate contact pressure. METHODS: We cyclically moved 10 fresh cadaver wrists using a wrist joint motion simulator while measuring the contact pressures between the proximal carpal row and the distal radius and ulna using a dynamic pressure sensor. We acquired data in the intact wrist, after a midcarpal arthrodesis with the scaphoid excised, and then again with the triquetrum removed, which is also known as a capitolunate arthrodesis. RESULTS: The peak pressures in the radiolunate fossa significantly increased with either of the midcarpal arthrodeses compared with the intact wrist during each of the 3 dynamic wrist motions. In comparing the 2 midcarpal arthrodeses, the peak pressure in the ulnocarpal fossa significantly decreased after the triquetrum was removed during wrist radioulnar deviation and in the static ulnarly deviated position. After arthrodesis, we could identify no differences during any motion or static wrist position in the peak radiolunate pressures with or without the triquetrum. CONCLUSIONS: We found that scaphoid excision and 4-corner arthrodesis shifts loads to the radiolunate joint. Isolated capitolunate arthrodesis with excision of the scaphoid and triquetrum further alters carpal kinematics and loading patterns. CLINICAL RELEVANCE: These findings raise concern about routine excision of the triquetrum when performing a midcarpal arthrodesis.

Does glenoid baseplate geometry affect its fixation in reverse shoulder arthroplasty?

BACKGROUND: The effect of glenoid baseplate geometry has not been studied as it pertains to reverse shoulder arthroplasty. The purpose of this study was to compare 2 baseplate designs whose major difference is being either a flat backed design or a convex baseplate, with regard to their bone interface area, screw engagement, and bone volume removed using 3-dimensional modeling. METHODS: Three-dimensional models of 6 scapulae were used to virtually implant models of a flat backed and a convex backed glenoid baseplate. Additional reaming was performed in 1 mm increments, up to 5 mm, and the amount of baseplate screw engagement was calculated at each increment. Statistical differences between flat and convex implants were calculated. RESULTS: Insertion of the convex baseplate required statistically greater removal of bone as compared to the flat baseplate (P = .003). No statistical changes in total area were observed with reaming of the glenoid for the convex baseplate (P > .095). However, for the flat baseplate, 1 mm of reaming caused a statistical decrease in area available for fixation. The amount of total bone area in contact with a convex baseplate was statistically greater than with a flat baseplate (P = .004). The amount of screw engagement was statistically less with the convex baseplate, compared to the flat (P = .026). DISCUSSION: A convex backed glenoid baseplate can improve the contact surface area at the bone implant interface as compared to a flat backed design. However, better screw engagement and less bone volume removed during reaming favors a flat backed design, particularly when adequate bone-implant contact cannot be achieved.

Structural properties of 6 forearm ligaments.

PURPOSE: To first determine the structural properties of 6 forearm ligaments and then to create linear and nonlinear analytical models of each ligament from these properties. METHODS: We nondestructively tested the annular ligament, dorsal and palmar radioulnar ligaments, and the distal, central, and proximal bands of the interosseous ligament from 7 fresh cadaver forearms in a servohydraulic testing apparatus. We performed testing with the bone-ligament-bone constructs positioned corresponding to neutral forearm rotation as well as in 45° of supination and 45° of pronation. Based on a mechanical creep test of each ligament, we computed a linear and nonlinear ligament stiffness value for each ligament. We then compared these computed analytical responses to loading with loading data when each ligament was tested at 1.0 and 0.05 mm/s. We analyzed differences among ligaments and forearm positions using 1-way and 2-way analyses of variance. RESULTS: The stiffnesses for the distal band and the dorsal radioulnar ligament were statistically less when the constructs were positioned in supination compared with neutral forearm rotation. At all forearm positions, the linear stiffness of the central band was greater than that for the distal band of the interosseous ligament, the proximal band of the interosseous ligament, and the dorsal radioulnar and palmar radioulnar ligaments. In neutral forearm rotation, the linear stiffness of the central band was statistically greater than the annular ligament. The experimental loading behavior of each ligament was better modeled by a nonlinear stiffness than a linear one. CONCLUSIONS: The central band of the interosseous membrane is the stiffest stabilizing structure of the forearm. Any structure used to replace the central band or other forearm ligaments should demonstrate a nonlinear response to loading. CLINICAL RELEVANCE: In considering a reconstruction for the forearm, the graft used should have a nonlinear response to loading and be one that is similar to the normal, original ligament.

Morphology of the cubital tunnel: an anatomical and biomechanical study with implications for treatment of ulnar nerve compression.

PURPOSE: The purpose of this study is to provide a thorough understanding of the anatomy of the cubital tunnel and to outline specific anatomical parameters of the cubital tunnel retinaculum (CuTR) that might aid in the management of ulnar nerve problems. The hypotheses of this study are (1) that the nerve elongates with elbow flexion and (2) that the cross-sectional area of the cubital tunnel is inversely proportional to the degree of elbow flexion. METHODS: Eleven fresh-frozen cadaver arms were dissected at the medial elbow. The CuTR was identified, and its thickness was measured. After excising the CuTR, we measured the elongation of the anterior and posterior aspects of the ulnar nerve, as well as the length of the CuTR origin/insertion, at increasing intervals of elbow flexion (15°, 30°, 45°, 90°, 120°, and 135°). Using 3-dimensional digitization technology, the surface of the cubital tunnel was recorded at 4 positions of elbow flexion (15°, 45°, 90°, and 135°) and analyzed to define the tunnel geometry. RESULTS: The CuTR origin-to-insertion length and the ulnar nerve length both increased significantly with increasing flexion angle. Both lengths at 90°, 120°, and 135° of elbow flexion were greater than at 15° or 30°. The cubital tunnel area was significantly less at 135° compared to either 45° or 90° of flexion. There was a linear relationship between the cubital tunnel area of the different arms with the corresponding nerve cross-sectional area when measured at the level of the epicondyle and when the arm was at 90° of elbow flexion. CONCLUSIONS: The CuTR begins to stretch at 60° of flexion and continues to stretch with increasing flexion. Similarly, the ulnar nerve is more taut in flexion. The area within the cubital tunnel decreases beyond 90° of elbow flexion. CLINICAL RELEVANCE: Understanding the dynamic anatomical relationships of the cubital tunnel might help in the safe treatment of cubital tunnel syndrome when using minimally invasive techniques and instrumentation.

Scaphoid and lunate translation in the intact wrist and following ligament resection: a cadaver study.

PURPOSE: To determine the amount of scaphoid and lunate translation that occurs in normal cadaver wrists during wrist motion, and to quantify the change in ulnar translation when specific dorsal and volar wrist ligaments were sectioned. METHODS: We measured the scaphoid and lunate motion of 37 cadaver wrists during wrist radioulnar deviation and flexion-extension motions using a wrist joint motion simulator. We quantified the location of the centroids of the bones during each motion in the intact wrists and after sectioning either 2 dorsal ligaments along with the scapholunate interosseous ligament or 2 volar ligaments and the scapholunate interosseous ligament. RESULTS: In the intact wrist, the scaphoid and lunate statistically translated radially with wrist ulnar deviation. With wrist flexion, the scaphoid moved volarly and the lunate dorsally. After sectioning either the dorsal or volar ligaments, the scaphoid moved radially. After sectioning the dorsal or volar ligaments, the lunate statistically moved ulnarly and volarly. CONCLUSIONS: Measurable changes in the scaphoid and lunate translation occur with wrist motion and change with ligament sectioning. However, for the ligaments that were sectioned, these changes are small and an attempt to clinically measure these translations of the scaphoid and lunate radiographically may be limited. The results support the conclusion that ulnar translocation does not occur unless multiple ligaments are sectioned. Injury of more than the scapholunate interosseous ligament along with either the dorsal intercarpal and dorsal radiocarpal or the radioscaphocapitate and scaphotrapezial ligaments is needed to have large amounts of volar and ulnar translation.

Wrist tendon forces during various dynamic wrist motions.

PURPOSE: A common treatment of arthritis of the thumb carpometacarpal joint requires all or a portion of the flexor carpi radialis tendon (FCR) to be used as an interpositional graft. The purpose of this study was to examine the in vitro tendon forces in 6 wrist flexors and extensors to determine whether their force contribution changes during various dynamic wrist motions along with a specific application to the FCR. METHODS: We tested 62 fresh-frozen cadaver wrists in a wrist joint motion simulator. During wrist flexion-extension, radioulnar deviation, dart throwing, and circumduction motions, the peak and average tendon forces were determined for the extensor carpi ulnaris, extensor carpi radialis brevis and longus, abductor pollicis longus, flexor carpi radialis, and flexor carpi ulnaris. RESULTS: During a dart-throwing motion, the mean and peak FCR forces were statistically less than during the other 3 motions. Conversely, the mean and peak flexor carpi ulnaris forces were statistically greater during the dart-throwing motion than during the other 3 motions. CONCLUSIONS: Patients who have undergone a surgical procedure in which all or a portion of the FCR has been harvested may experience a decrease in wrist strength with wrist motion, as the FCR tendon normally applies force during wrist motion. The motion least likely to be affected by such surgery is the dart-throwing motion when the force on the remaining FCR is minimized.

Forces across the middle of the intact clavicle during shoulder motion.

HYPOTHESIS: The optimal management of displaced middle third clavicle fractures is currently under investigation. Advances in fracture fixation technology have expanded the indications for operative fracture management. Data are currently unavailable regarding the normal forces and moments that occur in the middle clavicle with motion of the glenohumeral joint. This study tested our null hypothesis that active range of motion in internal rotation, external rotation, and abduction would produce a similar magnitude of force across the middle clavicle. MATERIALS AND METHODS: Clavicle forces were measured in 6 whole fresh frozen cadavers using a 6 degree-of-freedom load cell mounted to the middle third of the clavicle. The rotator cuff tendons were isolated, divided, and connected to a system of weights. The forces across the clavicle in 3 orthogonal directions were quantified during simulated active abduction, internal rotation, and external rotation. RESULTS: There were statistically greater axial compressive force and torque in the clavicle during humeral abduction compared with internal or external rotation. During external rotation, there were statistically greater tensile forces compared with abduction or internal rotation. There were no statistical differences in the superior-inferior or anterior-posterior forces with the 3 motions studied. DISCUSSION: Overall, active abduction caused the greatest increase in middle clavicle forces and torque. Abduction resulted in the most significant axial compressive force, whereas active external rotation caused the greatest tensile force across the intact middle clavicle. CONCLUSIONS: To our knowledge, these findings represent the first results describing the forces across the intact clavicle during glenohumeral motion. These data can be used to aid clinicians in treating these fractures, guide the design of future biomechanical studies, and develop rehabilitation protocols.

Optimization of glenoid fixation in reverse shoulder arthroplasty using 3-dimensional modeling.

BACKGROUND: Reverse shoulder fixation complications may be related to how much glenoid bone is removed and how the fixation screws are located in the glenoid. The purpose of this study was to determine how much bone volume and surface area are lost with incremental reaming and to evaluate screw fixation. METHODS: A contemporary reverse shoulder implant was virtually implanted into models of 6 different shoulders following initial and then after additional incremental reaming was performed. Changes in the glenoid bone surface area and volume available for fixation were statistically evaluated using repeated measures ANOVAs. RESULTS: The total bone volume, the amount of reamed glenoid surface area available for an implant baseplate, and the actual amount of the glenoid in contact with a baseplate decreased with increasing amounts of reaming. With 5 mm of reaming, the total volume decreased by 2810 mm(3), the reamed glenoid surface area decreased by 28%, and the amount of the glenoid in contact with the baseplate decreased by 27%. The amount of engagement of anterior and posterior screws was much less than that of the superior and inferior screws. CONCLUSION: Careful reaming of the glenoid surface is critical, because as little as 1 extra millimeter of bone removal decreases the amount of bone available for implant fixation. After reaming there may not be enough bone to accommodate anterior and posterior screws with the design used in this study.

Dynamic biomechanical evaluation of the dorsal intercarpal ligament repair for scapholunate instability.

PURPOSE: A variety of soft tissue surgical procedures have been developed for treatment of scapholunate dissociation. One reconstruction method, using the dorsal intercarpal ligament, has been used clinically with some success. The purpose of this study was to evaluate biomechanically use of the dorsal intercarpal ligament for static scapholunate dissociation. METHODS: Eight cadaver wrists were tested in a wrist joint motion simulator. Each wrist was moved in continuous cycles of flexion-extension and radial-ulnar deviation. Kinematic data for the scaphoid and lunate were recorded for each wrist in the intact state, after the scapholunate interosseous, dorsal radiocarpal, and dorsal intercarpal ligaments were sectioned, and after reconstruction using the dorsal intercarpal ligament. RESULTS: Ligamentous sectioning resulted in static scapholunate dissociation. Visually, the repair initially reduced the gap between the scaphoid and lunate, but within a few cycles of wrist motion, there were statistically significant increases in scaphoid flexion, scaphoid ulnar deviation, and lunate extension. In 6 arms, gapping between the scaphoid and lunate was observed. In 2 arms, a gap occurred and the repair also pulled out of the bone junction. CONCLUSIONS: This study does not support the hypothesis that the dorsal intercarpal ligament repair alone will stabilize the scaphoid and lunate after scapholunate instability in the immediate postoperative period.

Proximal row carpectomy: role of a radiocarpal interposition lateral meniscal allograft.

PURPOSE: The purpose of this study was to determine whether use of a lateral meniscal interposition allograft combined with proximal row carpectomy would reduce the peak joint contact pressures and increase the contact area when compared with proximal row carpectomy alone. METHODS: Six cadaver wrists were cyclically moved through flexion-extension and radioulnar deviation ranges of motion. Joint contact pressure was measured with the carpus intact, after proximal row carpectomy, and after insertion of a lateral meniscal allograft. Contact pressure data were also collected with the wrist in 5 static positions. RESULTS: Proximal row carpectomy caused statistically greater peak pressures and smaller contact areas when compared with the intact wrist. Insertion of the allograft statistically restored the pressures and areas to that observed in the intact wrist. CONCLUSIONS: These results support the clinical trial of a lateral meniscal interposition allograft in patients with contraindications for proximal row carpectomy, such as pre-existing arthritis in the capitate head or lunate facet of the radius.

Treatment of scapholunate dissociation with a bioresorbable polymer plate: a biomechanical study.

PURPOSE: Untreated scapholunate dissociation can lead to pain and eventually arthritis. There has been minimal limited success of soft tissue surgical procedures to repair or functionally replicate the torn structures. In this study, we evaluated a new surgical repair for scapholunate dissociation using a bioresorbable polymer approved for human applications. METHODS: Eight cadaver wrists were tested in a wrist joint motion simulator. Each wrist was moved in continuous cycles of flexion-extension and radioulnar deviation. Kinematic data for the scaphoid and lunate were recorded for each wrist in the intact state; after the scapholunate interosseous, dorsal radiocarpal, and dorsal intercarpal ligaments were sectioned; after repair using a 4-hole bioresorbable plate; and after 1,000 cycles of wrist motion to mimic continued use after surgery. RESULTS: Sectioning of these 3 wrist ligaments resulted in static scapholunate dissociation. Application of the polymer plate statistically restored the scaphoid and lunate kinematics to that of the intact specimen. Scapholunate instability and any gap between the bones was eliminated. After 1,000 cycles of motion, the plate maintained intact kinematics in 5 of 8 arms. During cyclic motion, either the plate failed or the screws pulled out in the remaining 3 arms. This occurred in the smallest and most osteoporotic cadavers in which positioning a sensor post and the 2 screws for the plate in the small lunate compromised the pullout strength of the screws. CONCLUSIONS: Use of a resorbable polymer plate in restoring normal kinematics in patients with scapholunate dissociation is supported by this study.

The Effect of Scaphoid Fracture Site on Scaphoid Instability Patterns.

Background Scaphoid fractures are common carpal fractures that are often misdiagnosed as wrist sprains and may go on to nonunion. The location of the fracture site may influence the stability of scaphoid nonunions. Purpose To determine whether the stability of a scaphoid nonunion depends upon the fracture's location, we tested the hypothesis that a simulated fracture distal to the apex of the scaphoid dorsal ridge will have greater interfragmentary motion than proximal. Methods Eleven cadaver wrists were moved through three wrist motions using a wrist simulator. In six wrists, a fracture was created distal to the scaphoid apex, and in five a fracture was created proximal to the apex. Sensors attached to the distal and proximal parts of each scaphoid measured the interfragmentary motion during wrist motion. Results In those wrists in which the scaphoid was sectioned distal to the apex, the distal fragment became significantly more unstable relative to the proximal fragment. It flexed, ulnarly deviated, and pronated. These motion changes were less when the scaphoid was sectioned proximally. Discussion Scaphoid fractures distal to the scaphoid apex will have greater interfragmentary motion. The mobility of the fragments at the fracture site is possibly a more important contributory factor of nonunion in scaphoid waist fractures than for proximal scaphoid fractures. Clinical Relevance Understanding the effect that the location of a scaphoid fracture has on the potential for nonunion may influence the modalities of treatment and follow-up.

A comparison of anterior cruciate ligament graft tunnel orientation: anatomic vs. transtibial.

BACKGROUND: Recent Anterior Cruciate Ligament reconstruction techniques have emphasized reproducing the insertion sites of the native Anterior Cruciate Ligament. Anatomic techniques have shown improvements in biomechanical testing, but their superior results have not been shown clinically. The hypothesis of this study is that more oblique tunnels utilized in anatomic reconstructions cause asymmetric loading across the graft. METHODS: Seven cadaver knees were tested in a knee simulator that performed a gait cycle and an anterior-posterior laxity test. Each knee underwent both reconstructions in random order utilizing the same Anterior Cruciate Ligament bone patellar tendon bone graft. Before reconstruction, the graft was split longitudinally and miniature force probes were inserted in the medial and lateral portions. FINDINGS: During anterior-posterior laxity testing, the transtibial medial bundle averaged 74.8N compared to 87N for the anatomic. The lateral bundles averaged 146.2 and 158N respectively. Both reconstructions exhibited a similar ratio of force distribution between the bundles and there was no statistical difference. The average anterior-posterior motion for the intact knees was 10.8mm compared to 17.0mm after the Anterior Cruciate Ligament was sectioned. Anatomic reconstructions had an average of 14.0mm of laxity compared to 14.9mm for transtibial reconstructions (P<0.038). INTERPRETATION: Greater obliquity did not lead to an increase in asymmetry of graft loading. The failure of anatomic reconstructions to show clinical improvement over transtibial reconstructions is not due to oblique tunnels causing asymmetric graft loading.

In vitro response of the natural cadaver knee to the loading profiles specified in a standard for knee implant wear testing.

The purpose of this study was to examine how a natural knee responds to the inputs of a total knee replacement testing standard developed by the International Organization for Standardization (ISO). This load control standard prescribes forces to be used for wear testing of knee replacements independent of implant size or design. A parallel ISO standard provides wear testing inputs that are displacement based instead of force based. Eight fresh frozen cadaveric knees were potted and tested in a 6 degree of freedom knee simulator using the load-control standard. The resulting displacements during load-control testing were compared to the prescribed displacements of the ISO displacement standard. At half the tibial torque prescribed by the load standard there was three times more average internal tibial rotation (20.3 degrees) than is prescribed by the displacement standard (5.7 degrees). The AP motion resulting from load testing was much different than is specified by the displacement standard. All eight knees had anterior tibial translation with respect to the femur during swing phase while the displacement standard specifies posterior tibial displacement. The variation in these motions among knees and their difference from the ISO displacement standard may be one factor that explains why wear results of total knee replacements based on ISO load or displacement testing frequently do not agree with each other or with clinical retrievals.

Results of laboratory evaluation of acute knee effusion after anterior cruciate ligament reconstruction: what is found in patients with a noninfected, painful postoperative knee?

BACKGROUND: Infection after anterior cruciate ligament reconstruction is a rare and potentially devastating complication. No normative data have been reported for knee aspiration after anterior cruciate ligament reconstruction in the early postoperative period. HYPOTHESIS: Determining normative laboratory data from a retrospective review of noninfected early postoperative anterior cruciate ligament reconstruction knee effusions will allow for the calculation of an aspirate white blood cell (WBC) threshold value indicative of infection. STUDY DESIGN: Case series (diagnosis); Level of evidence, 4. METHODS: A 2-year retrospective chart review of 151 anterior cruciate ligament reconstruction patients was performed. Thirty-one noninfected patients meeting the inclusion and exclusion criteria and 1 infected patient had laboratory data collected, including peripheral blood and knee effusion aspirate analyses. Laboratory data from pertinent published studies of infected knees after anterior cruciate ligament reconstruction were combined with the data of our 1 infected patient, establishing a historical control group. Data were analyzed and results were then compared. Infected aspirate WBC threshold value statistics were then calculated. RESULTS: Analysis of noninfected knee effusion aspirates revealed a mean WBC count of 9600/uL (standard deviation [SD], 15 200), and a mean of 66% polymorphonuclear (PMN) cells (SD, 34). Aspirate WBC 98% confidence interval (CI) was 2800/uL to 16 200/uL, and the 98% CI for PMN cells was 58% to 84%. Aspirate WBC count >16 200/uL is 86% sensitive, 92% specific, and has a positive likelihood ratio of 10.4 as an indicator of infection. CONCLUSION: Benign effusion after anterior cruciate ligament reconstruction is common and is associated with elevated inflammatory markers. When concerned, knee aspiration after anterior cruciate ligament surgery gives the highest yield to differentiate between a painful effusion and a septic knee in the early postoperative period while awaiting definitive culture results. The authors report confidence intervals defining the range of cell count variables for noninfected patients requiring aspiration, specifically WBC and PMN, and suggest a WBC threshold value of >16 200/uL be used as an indicator of infection. On the basis of comparison with historical control data, the authors believe these data are significant and will be reliable for clinical use.