An Anterior Spike Decreases Bone-Implant Micromotion in Cementless Tibial Baseplates for Total Knee Arthroplasty: A Biomechanical Study.

BACKGROUND: Cementless tibial baseplates in total knee arthroplasty include fixation features (eg, pegs, spikes, and keels) to ensure sufficient primary bone-implant stability. While the design of these features plays a fundamental role in biologic fixation, the effectiveness of anterior spikes in reducing bone-implant micromotion remains unclear. Therefore, we asked: Can an anterior spike reduce the bone-implant micromotion of cementless tibial implants? METHODS: We performed computational finite element analyses on 13 tibiae using the computed tomography scans of patients scheduled for primary total knee arthroplasty. The tibiae were virtually implanted with a cementless tibial baseplate with 2 designs of fixation of the baseplate: 2 pegs and 2 pegs with an anterior spike. We compared the bone-implant micromotion under the most demanding loads from stair ascent between both designs. RESULTS: Both fixation designs had peak micromotion at the anterior-lateral edge of the baseplate. The design with 2 pegs and an anterior spike had up to 15% lower peak micromotion and up to 14% more baseplate area with micromotions below the most conservative threshold for ingrowth, 20 µm, than the design with only 2 pegs. The greatest benefit of adding an anterior spike occurred for subjects who had the smallest area of tibial bone below the 20 µm threshold (ie, most at risk for failure to achieve bone ingrowth). CONCLUSIONS: An anteriorly placed spike for cementless tibial baseplates with 2 pegs can help decrease the bone-implant micromotion during stair ascent, especially for subjects with increased bone-implant micromotion and risk for bone ingrowth failure.

Is Tibial Bone Mineral Density Related to Sex, Age, Preoperative Alignment, or Fixation Method in Primary Total Knee Arthroplasty?

BACKGROUND: Cementless total knee arthroplasty (TKA) has regained interest for its potential for long-term biologic fixation. The density of the bone is related to its ability to resist static and cyclic loading and can affect long-term implant fixation; however, little is known about the density distribution of periarticular bone in TKA patients. Thus, we sought to characterize the bone mineral density (BMD) of the proximal tibia in TKA patients. METHODS: We included 42 women and 50 men (mean age 63 years, range: 50 to 87; mean body mass index 31.6, range: 20.5 to 49.1) who underwent robotic-assisted TKA and had preoperative computed tomography scans with a BMD calibration phantom. Using the robotic surgical plan, we computed the BMD distribution at 1 mm-spaced cross-sections parallel to the tibial cut from 2 mm above the cut to 10 mm below. The BMD was analyzed with respect to patient sex, age, preoperative alignment, and type of fixation. RESULTS: The BMD decreased from proximal to distal. The greatest changes occurred within ± 2 mm of the tibial cut. Age did not affect BMD for men; however, women between 60 and 70 years had higher BMD than women ≥ 70 years for the total cut (P = .03) and the medial half of the cut (P = .03). Cemented implants were used in 1 86-year-old man and 18 women (seven < 60 years, seven 60 to 70 years, and four ≥ 70 year old). We found only BMD differences between cemented or cementless fixation for women < 60 years. CONCLUSIONS: To our knowledge, this is the first study to characterize the preoperative BMD distribution in TKA patients relative to the intraoperative tibial cut. Our results indicate that while sex and age may be useful surrogates of BMD, the clinically relevant thresholds for cementless knees remain unclear, offering an area for future studies.

Undersizing the Tibial Baseplate in Cementless Total Knee Arthroplasty has Only a Small Impact on Bone-Implant Interaction: A Finite Element Biomechanical Study.

BACKGROUND: The tibial component in total knee arthroplasty (TKA) is often chosen to maximize coverage of the tibial cut, which can result in excessive internal rotation of the component. Optimal rotational alignment may require a smaller baseplate with suboptimal coverage that could threaten fixation. We asked: "does undersizing the tibial component of a cementless TKA to gain external rotation increase the risk of bone failure?" METHODS: We developed computational finite element (FE) analysis models from the computed tomography (CT) scans of 12 patients scheduled for primary TKA. The models were implanted with a cementless tibial baseplate that maximized coverage and one or two sizes smaller and externally rotated by 5°. We calculated the risk of bone collapse under loads representative of stair ascent. RESULTS: Undersizing the implant increased the area at risk of collapse for eight patients. However, the area at risk of collapse for the undersized implant (range, 5.2%-16.4%) was no different (P = .24) to the optimally sized implant (range, 4.5%-17.9%). The bone at risk of collapse was concentrated along the posterior edge of the implant. The area at risk of collapse was not proportional to implant size, and for four subjects undersizing the implant actually decreased the area at risk of collapse. CONCLUSION: While implants should maximize coverage of the tibial cut and seek support on dense bone, undersizing the tibial component to gain external rotation had minimal impact on the load transfer to the underlying bone. This FE analysis model of a cementless tibial baseplate may require further validation and additional studies to investigate the long-term biomechanical effects of undersizing the tibial baseplate. In conclusion, while surgeons should strive to use the appropriate tibial baseplate for each patient, our model identified only minor biomechanical consequences of undersizing the implant for the immediate postoperative bone-implant interaction and implant subsidence.

Do Metaphyseal Cones and Stems Provide Any Biomechanical Advantage for Moderate Contained Tibial Defects in Revision TKA? A Finite-Element Analysis Based on a Cadaver Model.

BACKGROUND: Satisfactory management of bone defects is important to achieve an adequate reconstruction in revision TKA. Metaphyseal cones to address such defects in the proximal tibia are increasingly being used; however, the biomechanical superiority of cones over traditional techniques like fully cementing the implant into the defect has not yet been demonstrated. Moreover, although long stems are often used to bypass the defects, the biomechanical efficacy of long stems compared with short, cemented stems when combined with metaphyseal cones remains unclear. QUESTIONS/PURPOSES: We developed and validated finite-element models of nine cadaveric specimens to determine: (1) whether using cones for addressing moderate metaphyseal tibial defects in revision TKA reduces the risk of implant-cement debonding compared with cementing the implant alone, and (2) when using metaphyseal cones, whether long, uncemented stems (or diaphyseal-engaging stems) reduce the risk of implant-cement debonding and the cone-bone micromotions compared with short, cemented stems. METHODS: We divided nine cadaveric specimens (six male, three female, aged 57 to 73 years, BMI 24 to 47 kg/m2) with standardized tibial metaphyseal defects into three study groups: no cone with short (50-mm) cemented stem, in which the defect was filled with cement; cone with short (50-mm) cemented stem, in which a metaphyseal cone was implanted before cementing the implant; and cone with long, diaphyseal-engaging stem, which received a metaphyseal cone and the largest 150-mm stem that could fit the diaphyseal canal. The specimens were implanted and mechanically tested. Then, we developed and validated finite-element models to investigate the interaction between the implant and the bone during the demanding activity of stair ascent. We quantified the risk of implant debonding from the cement mantle by comparing the axial and shear stress at the cement-implant interface against an experimentally derived interface failure index criterion that has been previously used to quantify the risk of cement debonding. We considered the risk of debonding to be minimal when the failure index was below 10% of the strength of the interface (or failure index < 0.1). We also quantified the micromotion between the cone and the bone, as a guide to the likelihood of fixation by bone ingrowth. To this end, we assumed bone ingrowth for micromotion values below the most restrictive reported threshold for bone ingrowth, 20 µm. RESULTS: When using a short, 50-mm cemented stem and cement alone to fill the defect, 77% to 86% of the cement-implant interface had minimal risk of debonding (failure index < 0.1). When using a short, 50-mm cemented stem with a cone, 87% to 93% of the cement-implant interface had minimal debonding risk. When combining a cone with a long (150-mm) uncemented stem, 92% to 94% of the cement-implant interface had minimal debonding risk. The differences in cone-bone micromotion between short, cemented stems and long, uncemented stems were minimal and, for both configurations, most cones had micromotions below the most restrictive 20-µm threshold for ingrowth. However, the maximum micromotion between the cone and the bone was in general smaller when using a long, uncemented stem (13-23 µm) than when using a short, cemented stem (11-31 µm). CONCLUSION: Although the risk of debonding was low in all cases, metaphyseal cones help reduce the biomechanical burden on the implant-cement interface of short-stemmed implants in high-demand activities such as stair ascent. When using cones in revision TKA, long, diaphyseal-engaging stems did not provide a clear biomechanical advantage over short stems. Future studies should explore additional loading conditions, quantify the interspecimen variability, consider more critical defects, and evaluate the behavior of the reconstructive techniques under repetitive loads. CLINICAL RELEVANCE: Cones and stems are routinely used to address tibial defects in revision TKA. Despite our finding that metaphyseal cones may help reduce the risk of implant-cement debonding and allow using shorter stems with comparable biomechanical behavior to longer stems, either cones or cement alone can provide comparable results in contained metaphyseal defects. However, longer term clinical studies are needed to compare these techniques over time.

The Use of Three-Dimensional Printing for Complex Scaphoid Fractures.

Scaphoid fractures are one of the most common fractures treated by hand surgeons. The complex anatomy and size of the scaphoid bone can make the reduction and fixation of these fractures technically challenging. Careful preoperative planning is required to ensure stable fixation is achieved. We report on the use of 3-dimensional printing to improve preoperative planning for a series of complex scaphoid fractures.

A Comparison Between Chinese and Caucasian 3-Dimensional Bony Morphometry in Presimulated and Postsimulated Osteotomy for Total Knee Arthroplasty.

BACKGROUND: The bone morphologies of intact knees were measured and compared between Chinese and Caucasian populations. However, to assess if distinct designs of implants are necessary for the Chinese population owing to different morphologies and sizes, the knee measurements after osteotomy performed in total knee arthroplasty were evaluated. METHODS: Thirty-seven Caucasian and 50 Chinese patients' knees were examined using computed tomography scans. Mimics were applied to reconstruct 3-dimensional bone models. Dimensions of the 3-dimensional knee models and simulated bone resections during total knee arthroplasty were measured using Geomagic Studio and Pro/ENGINEER. The morphologic measurements of the native and resected femur and tibia included the anteroposterior (AP) depth, mediolateral (ML) width, notch width, knee physical valgus angle, tibial slope angle, and the ML-to-AP ratio of the femur, tibia, and resected femur. Statistical analysis was performed using the independent samples t test and the Pearson correlation coefficient in SPSS for Windows. Values of P < .05 were considered significant. RESULTS: No measurements were significantly different between the Chinese and Caucasian knees. However, the Chinese female showed significant differences compared with the Chinese male on distal femoral measurements both presimulated and postsimulated osteotomy such as a smaller mean ML-to-AP ratio in presimulated (1.3 ± 0.1) and postsimulated (1.3 ± 0.1) osteotomy. CONCLUSION: The necessity of designing a full set of total knee components specifically for the Chinese population is still undetermined. However, we suggest designing femoral components specific for the Chinese females because of different postosteotomy distal femoral ML-to-AP ratio between the Chinese males and the Chinese females.

MRI of Polyethylene Tibial Inserts in Total Knee Arthroplasty: Normal and Abnormal Appearances.

OBJECTIVE: The purpose of this study was to describe the normal and abnormal MRI appearances of polyethylene tibial inserts. Subjects who underwent MRI before revision total knee arthroplasty were identified. The polyethylene tibial insert's shape was categorized on MRI, and the presence of abnormalities was noted. CONCLUSION: The shape of the polyethylene tibial insert varied with the design. Polyethylene tibial insert abnormalities seen on MRI included displacement and fracture. MRI distinguishes various designs of polyethylene tibial inserts and can show associated abnormalities.

Effect of Spinal Deformity on Pelvic Orientation from Standing to Sitting Position.

BACKGROUND: The effect of fixed spinal deformities on a functional pelvis from standing to sitting is not fully understood. We aimed to assess the change in preoperative sagittal pelvic tilt angle (SPTA) from standing to sitting in patients undergoing total hip arthroplasty, comparing flexible and fixed spinal deformities. METHODS: Between July 2011 and October 2011, 68 consecutive unilateral total hip arthroplasties were implanted in 68 patients with a mean age of 71 ± 6 years. Fixed spinal deformity was defined as <10° of the change in SPTA from standing to sitting. Preoperative radiographic evaluation included standing (weight-bearing) anteroposterior and lateral pelvic and lumbosacral radiographs and a sitting lateral pelvic radiograph. RESULTS: The mean standing and sitting SPTA was 3.7° of anterior tilt and 17.7° of posterior tilt, respectively (change of 21.4 ± 12.5°). Seventy-five percent had flexible pelvises, all of which had a posterior tilt from standing to sitting. One patient in the fixed pelvis (1.4%) had a loss of posterior tilt from standing to sitting. The mean change of SPTA from standing to sitting in the fixed and flexible pelvis groups was 5.9 ± 3.5° to 26.7 ± 9.6° of posterior tilt, which was statistically significant (P < .05). CONCLUSION: There was a significant change in sagittal pelvic tilt from standing to sitting, especially in patients with a flexible spine, in which the functional anteversion increases with sitting. The patients with a fixed pelvis had significantly less SPTA in standing (less anteversion) with less posterior sagittal tilt in sitting, which should be incorporated in cup positioning.

Frontal plane stability following UKA in a biomechanical study.

INTRODUCTION: Function and kinematics following unicondylar knee arthroplasty (UKA) have been reported to be close to the native knee. Gait, stair climbing and activities of daily living expose the knee joint to a combination of varus and valgus moments. Replacement of the medial compartment via UKA is likely to change the physiologic knee stability and its ability to respond to varus and valgus moments. It was hypothesized that UKA implantation would stiffen the knee and decrease range of motion in the frontal plane. MATERIALS AND METHODS: Six fresh frozen cadaver knees were prepared and mounted in a six-degrees-of-freedom robot. An axial load of 200 N was applied with the knee in 15°, 45° and 90° of flexion. Varus and valgus moments were added, respectively, before and after implantation of medial UKA. Tests were than redone with a thicker polyethylene inlay to simulate overstuffing of the medial compartment. Range of motion in the frontal plane and the tibial response to moments were recorded via the industrial robot. RESULTS: The range of motion in the frontal plane was decreased with both, balanced and overstuffed UKA and shifted towards valgus. When exposed to valgus moments, knees following UKA were stiffer in comparison with the native knee. The effect was even more pronounced with medial overstuffing. CONCLUSION: In UKA, the compressive anatomy is replaced by much stiffer components. This lack of medial compression and relative overstuffing leads to a tighter medial collateral ligament. This drives the trend towards a stiffer joint as documented by a decrease in frontal plane range of motion. Overstuffing should strictly be avoided when performing UKA.

Results of custom-fit, noncemented, semiconstrained total elbow arthroplasty for inflammatory arthritis at an average of eighteen years of follow-up.

BACKGROUND: The literature available on the results after noncemented total elbow arthroplasty (TEA) in inflammatory arthritis is limited. METHODS: Ten patients (7 women, 3 men; 14 elbows total) who underwent custom, noncemented TEA from 1988 to 1995 were retrospectively reviewed. The average age was 28 years (range, 17-45 years). Four patients (4 elbows) had rheumatoid arthritis, and 6 patients (10 elbows) had juvenile rheumatoid arthritis. The mean follow-up was 18 years. All patients underwent a custom, noncemented, semiconstrained TEA with a plasma spray surface designed from preoperative computed tomography scan to achieve metaphyseal fit. The primary outcome was the Mayo Elbow Performance Score, and secondary outcomes were flexion and rotation arc of motion. Intraoperative and postoperative complications and revisions performed were also recorded. Radiographs taken at final follow-up were evaluated for evidence of loosening. RESULTS: The Mayo Elbow Performance Score improved from a mean of 35 preoperatively to a mean of 91 postoperatively. Flexion arc of motion improved from 50° preoperatively to 111° postoperatively, and rotation arc improved from 75° preoperatively to 145° postoperatively. Four patients underwent bushing revision at 8, 8, 22, and 22 years (29%), respectively, and there was 1 deep infection (7%). One patient had an intraoperative fracture in the humerus that did not require further treatment. On final radiographic follow-up at a mean of 18 years, all the components were fully ingrown, and there was no evidence of loosening or loss of fixation. CONCLUSION: In the younger population with inflammatory arthritis, noncemented TEA has reliable outcomes clinically and radiographically at long-term follow-up.

A novel computational workflow to holistically assess total knee arthroplasty biomechanics identifies subject-specific effects of joint mechanics on implant fixation.

Computational studies of total knee arthroplasty (TKA) often focus on either joint mechanics (kinematics and forces) or implant fixation mechanics. However, such disconnect between joint and fixation mechanics hinders our understanding of overall TKA biomechanical function by preventing identification of key relationships between these two levels of TKA mechanics. We developed a computational workflow to holistically assess TKA biomechanics by integrating musculoskeletal and finite element (FE) models. For our initial study using the workflow, we investigated how tibiofemoral contact mechanics affected the risk of failure due to debonding at the implant-cement interface using the four available subjects from the Grand Challenge Competitions to Predict In Vivo Knee Loads. We used a musculoskeletal model with a 12 degrees-of-freedom knee joint to simulate the stance phase of gait for each subject. The computed tibiofemoral joint forces at each node in contact were direct inputs to FE simulations of the same subjects. We found that the peak risk of failure did not coincide with the peak joint forces or the extreme tibiofemoral contact positions. Moreover, despite the consistency of joint forces across subjects, we observed important variability in the profile of the risk of failure during gait. Thus, by a combined evaluation of the joint and implant fixation mechanics of TKA, we could identify subject-specific effects of joint kinematics and forces on implant fixation that would otherwise have gone unnoticed. We intend to apply our workflow to evaluate the impact of implant alignment and design on TKA biomechanics.

Hip joint center lateralization minimally affects the biomechanics of patient-specific flanged acetabular components: A computational model.

Patient-specific flanged acetabular components are utilized to treat failed total hip arthroplasties with large acetabular defects. Previous clinical studies from our institution showed that these implants tend to lateralize the acetabular center of rotation. However, the clinical impact of lateralization on implant survivorship is debated. Our goal was to develop a finite element model to quantify how lateralization of the native hip center affects periprosthetic strain and implant-bone micromotion distributions in a static level gait loading condition. To build the model, we computationally created a superomedial acetabular defect in a computed tomography 3D reconstruction of a native pelvis and designed a flanged acetabular implant to address this simulated bone defect. We modeled two implants, one with ~1 cm and a second with ~2 cm of hip center lateralization. We applied the maximum hip contact force and corresponding abductor force observed during level gait. The resulting strains were compared to bone fatigue strength (0.3% strain) and the micromotions were compared to the threshold for bone ingrowth (20 µm). Overall, the model demonstrated that the additional lateralization only slightly increased the area of bone at risk of failure and decreased the areas compatible with bone ingrowth. This computational study of patient-specific acetabular implants establishes the utility of our modeling approach. Further refinement will yield a model that can explore a multitude of variables and could be used to develop a biomechanically-based acetabular bone loss classification system to guide the development of patient-specific implants in the treatment of large acetabular bone defects.

Surgical technique: Computer-generated custom jigs improve accuracy of wide resection of bone tumors.

BACKGROUND: Manual techniques of reproducing a preoperative plan for primary bone tumor resection using rudimentary devices and imprecise localization techniques can result in compromised margins or unnecessary removal of unaffected tissue. We examined whether a novel technique using computer-generated custom jigs more accurately reproduces a preoperative resection plan than a standard manual technique. DESCRIPTION OF TECHNIQUE: Using CT images and advanced imaging, reverse engineering, and computer-assisted design software, custom jigs were designed to precisely conform to a specific location on the surface of partially skeletonized cadaveric femurs. The jigs were used to perform a hemimetaphyseal resection. METHODS: We performed CT scans on six matched pairs of cadaveric femurs. Based on a primary bone sarcoma model, a joint-sparing, hemimetaphyseal wide resection was precisely outlined on each femur. For each pair, the resection was performed using the standard manual technique on one specimen and the custom jig-assisted technique on the other. Superimposition of preoperative and postresection images enabled quantitative analysis of resection accuracy. RESULTS: The mean maximum deviation from the preoperative plan was 9.0 mm for the manual group and 2.0 mm for the custom-jig group. The percentages of times the maximum deviation was greater than 3 mm and greater than 4 mm was 100% and 72% for the manual group and 5.6% and 0.0% for the custom-jig group, respectively. CONCLUSIONS: Our findings suggest that custom-jig technology substantially improves the accuracy of primary bone tumor resection, enabling a surgeon to reproduce a given preoperative plan reliably and consistently.

Computerized range of motion analysis following dual mobility total hip arthroplasty, traditional total hip arthroplasty, and hip resurfacing.

Newer arthroplasty designs claim to provide superior range of motion (ROM) and greater stability than their predecessors. However, there is no way to compare ROM of implant systems in an equivalent anatomical environment in a clinical setting. This study used computer-aided design to compare ROM after hip resurfacing, 28 mm THA, 36 mm THA, and anatomic dual mobility (ADM) THA in 3D models of 5 cadaver pelvises. ROM to impingement was then tested in 10 different motions and a one-way ANOVA was used to compare results. The hip resurfacing resulted in restricted ROM compared to the other 3 models in all motions except adduction. The ADM, 36 mm, and 28 mm THA resulted in similar ROM. Dual mobility constructs provide comparable ROM in patients where large head THA is not appropriate.

Effect of varus alignment on the bone-implant interaction of a cementless tibial baseplate during gait.

Component alignment in total knee arthroplasty is a determining factor for implant longevity. Mechanical alignment, which provides balanced load transfer, is the most common alignment strategy. However, a retrospective review found that varus alignment, which could lead to unbalanced loading, can happen in up to 18% of tibial baseplates. This may be particularly burdensome for cementless tibial baseplates, which require low bone-implant micromotion and avoidance of bone overload to obtain bone ingrowth. Our aim was to assess the effect of varus alignment on the bone-implant interaction of cementless baseplates. We virtually implanted 11 patients with knee OA with a modern cementless tibial baseplate in mechanical alignment and in 2° of tibial varus alignment. We performed finite element simulations throughout gait, with loading conditions derived from literature. Throughout the stance phase, varus alignment had greater micromotion and percentage of bone volume at risk of failure than mechanical alignment. At mid-stance, when the most critical conditions occurred, the average increase in peak micromotion and amount of bone at risk of failure due to varus alignment were 79% and 59%, respectively. Varus alignment also resulted in the decrease of the surface area with micromotion compatible with bone ingrowth. However, for both alignments, this surface area was larger than the average area of ingrowth reported for well-fixed implants retrieved post-mortem. Our findings suggest that small varus deviations from mechanical alignment can adversely impact the biomechanics of the bone-implant interaction for cementless tibial baseplates during gait; however, the clinical implications of such changes remain unclear.

Comparison of anterior cruciate ligament tunnel position and graft obliquity with transtibial and anteromedial portal femoral tunnel reaming techniques using high-resolution magnetic resonance imaging.

PURPOSE: Using 3-dimensional high-resolution magnetic resonance imaging (MRI), we sought to compare femoral and tibial tunnel position and resultant graft obliquity with single-bundle anterior cruciate ligament (ACL) reconstruction using transtibial (TT) or anteromedial (AM) portal femoral tunnel reaming techniques. METHODS: Thirty patients were prospectively enrolled after primary, autogenous bone-patellar tendon-bone ACL reconstruction by 2 groups of high-volume, fellowship-trained sports medicine surgeons. With the TT technique, an external starting point was used to maximize graft obliquity and femoral footprint capture. By use of high-resolution MRI and imaging analysis software, bilateral 3-dimensional knee models were created, mirrored, and superimposed. Differences between centroids for each femoral and tibial insertion, as well as corresponding ACL/graft obliquity, were evaluated with paired t tests and 2-sided Mann-Whitney nonparametric tests, with P < .05 defined as significant. RESULTS: No significant differences were observed between groups in position of reconstructed femoral footprints. However, on the tibial side, AM centroids averaged 0.8 ± 1.9 mm anterior to native ACL centroids, whereas the TT group centered 5.23 ± 2.4 mm posterior to native ACL centroids (P < .001). Sagittal obliquity was closely restored with the AM technique (mean, 52.2° v. 53.5° for native ACL) but was significantly more vertical (mean, 66.9°) (P = .0001) for the TT group. CONCLUSIONS: In this clinical series, AM portal femoral tunnel reaming more accurately restored native ACL anatomy than the TT technique. Although both techniques can capture the native femoral footprint with similar accuracy, the TT technique requires significantly greater posterior placement of the tibial tunnel, resulting in decreased sagittal graft obliquity. When a tibial tunnel is drilled without the need to accommodate subsequent femoral tunnel reaming, more accurate tibial tunnel position and resultant sagittal graft obliquity are achieved. LEVEL OF EVIDENCE: Level III, retrospective comparative study.

Unicondylar knee retrieval analysis.

Unicondylar knee arthroplasty (UKA) is considered an alternative to total knee arthroplasty for patients who have arthritis limited to one compartment of the knee. This study examined surface damage of 3 contemporary UKA designs that were retrieved at revision surgery. Two of the UKA designs were fixed bearing and one was mobile bearing. Demographic information was collected, as well as information about the implants used at revision surgery. Articular surface damage was greater in the fixed-bearing designs as compared to the mobile bearing, although the mobile-bearing implants had significantly shorter length of implantation. Backside damage was also graded for the mobile bearing and when combined with articular wear resulted in overall damage scores higher than both fixed-bearing designs. The fixed-bearing designs showed delamination and surface deformation, whereas the mobile bearing had no evidence of these damage modes. However, mobile-bearing components showed other types of wear, and significant wear damage was present on the bearing surfaces of the mobile-bearing implants despite a short time of implantation. At the time of conversion to a total knee arthroplasty, more than 50% of cases required the use of stems, augments, or constrained inserts for the tibial reconstruction. In conclusion, wear modes differed among UKA prosthesis designs. Revision of a UKA to a total knee arthroplasty remains complex with the tibial preparation more complicated than in the primary setting.

Treatment of Recurrent Dislocation after Total Hip Arthroplasty Using Advanced Imaging and Three-Dimensional Modeling Techniques: A Case Series.

BACKGROUND: Surgical treatment options for addressing recurrent dislocation after total hip arthroplasty (THA) vary. Identifying impingement mechanisms in an unstable THA may be beneficial in determining appropriate treatment. QUESTIONS/PURPOSES: We sought to assess the effectiveness of developing pre-operative plans for treating hip instability after THA. We used advanced imaging and three-dimensional modeling techniques to perform impingement analyses in patients with unstable THA. METHODS: We evaluated a series of eight patients who would require revision THA to treat recurrent dislocation. Using a pre-operative algorithmic approach, we built patient-specific models and evaluated hip range of motion with computed tomographic scanning and biplanar radiography. This information was used to determine a surgical treatment plan that was then executed intra-operatively. Patients were followed for 2 years to determine whether they experienced another hip dislocation following treatment. RESULTS: Pre-operative kinematic modeling showed four of the eight patients had limited hip range of motion during flexion and internal rotation; a prominent anterior inferior iliac spine (AIIS) was found to limit hip range of motion in some of these cases. In the other four patients, range of motion was acceptable, suggesting soft-tissue causes of dislocation. No patients in this series experienced dislocation after undergoing revision THA. CONCLUSION: Advanced modeling techniques may be useful for identifying the impingement mechanisms responsible for instability after THA. Once variables contributing to limited hip range of motion are identified, surgeons can develop treatment plans to improve patient outcomes. Resecting a hypertrophic AIIS may improve hip range of motion and may be an important consideration for hip surgeons when revising unstable THAs.

Biplanar Low-Dose Radiography Is Accurate for Measuring Combined Anteversion After Total Hip Arthroplasty.

BACKGROUND: Acetabular component position alone has not been predictive of stability after total hip arthroplasty (THA). Combined anteversion of the acetabulum and femur has the potential of being more predictive of stability. Unfortunately, femoral component position is difficult to measure on plain radiographs. Computed tomography (CT) is the gold standard for measuring implant position post-operatively, but CT exposes patients to a substantial amount of radiation. QUESTIONS/PURPOSES: We sought to determine whether biplanar low-dose radiography can be used to accurately measure both acetabular and femoral implant position after THA. METHODS: Twenty patients underwent standing low-dose biplanar spine-to-ankle radiographs and supine CT scans 6 weeks after THA. Measurements of acetabular inclination, acetabular anteversion, and femoral anteversion were performed by two blinded observers and compared. RESULTS: The average absolute differences between biplanar radiographs and CT scans were 2° ± 2° for acetabular inclination, 3° ± 2° for acetabular anteversion, and 4° ± 4° for femoral anteversion between EOS measurements and CT measurements. Interobserver agreement was good for acetabular inclination, acetabular anteversion, and femoral anteversion (Cronbach's α = 0.90) using biplanar low-dose imaging. CONCLUSION: Biplanar radiography is a reliable low-radiation alternative for measuring acetabular inclination, acetabular anteversion, femoral version, and thus combined anteversion compared to CT. Femoral anteversion had the most variability but is still clinically relevant.