Biomechanical consequences of tibial tubercle osteotomy in a 3D-printed model of patellofemoral dysplasia (207)

Objectives: Trochlear dysplasia and an increased tibial tubercle-trochlear groove (TT-TG) distance are two major contributing factors to patellar instability and are often found concurrently. Patellar morphology is also abnormal in the setting of trochlear dysplasia. Indications for tibial tubercle osteotomy (TTO) include recurrent patellar instability in the setting of an increased TT-TG distance. While anteromedialization (AMZ) TTO has been shown to decrease overall PF contact stresses and improve patellar tracking, this has never been demonstrated in a model of PF dysplasia. Due in part to a lack of available dysplastic cadaveric specimens, few studies have investigated the consequences of PF dysplasia on PF biomechanics. Our previous work has demonstrated that when compared to normal morphology, PF dysplasia results in a lateral shift but negligible increases in patellar contact forces. This prompted the question of how TTO affects contact mechanics in this setting. The objective of this study was to quantify contact mechanics and kinematics following TTO using a 3D-printed PF dysplasia model. We hypothesized that an anterior tubercle position simulating AMZ TTO would best improve PF contact mechanics. Methods: Five fresh frozen cadaveric knees were dissected free of all soft tissues except the extensor mechanism. Computed tomography (CT) scan of each specimen confirmed no trochlear dysplasia or patella alta and a normal TT-TG distance (<10 mm). Dysplastic bone geometries were derived from patient CT scans selected by the senior orthopaedic surgeon who specializes in PF surgery. Segmentation was performed using Mimics (Materialise Figure 1A&B). Cadaveric knees were grouped based on the medial and lateral epicondylar distance (ML distance), and the implants were scaled to the size of each group. Scaling was done using Geomagic Studio (3D Systems), and implants were printed using a Form2 SLA 3D printer (Formlabs). Durable resin (Formlabs) was used to minimize wear between the printed components (Figure 1C). Cadaveric bony resection was performed using Biomet Vanguard (Zimmer Biomet) equipment. The amount of bone resected matched the 3D implant dimensions. A 6° distal femoral valgus cut angle was utilized. For femoral rotation, posterior referencing was utilized (no lateral insufficiency was observed), and cuts were made with 3° of external rotation in relation to the transepicondylar axis. The 3D implant was then fixed flush to the distal femur and native trochlea using screws. A metered patellar reamer was used for patellar preparation. The patellar implant was pressed into a central peg hole and fixed with a screw placed through the anterior patella. A flat tibial tubercle osteotomy cut, matching the aforementioned femoral rotation, was made with a shingle thickness of 1 cm and length of 6 cm. Each knee was mounted to a custom fixture on a servo-hydraulic load frame (MTS, Eden Prairie, MN) and cycled 5 times from 0° to 70° by pulling on the quadriceps tendon using a pulley system (Figure 1D). The shingle was fixed to the tibia using two 1.57mm K-wires. For each specimen, testing was repeated for each of three tibial tubercle positions: Native tubercle position (“normal”), 1 cm lateral to native (“lateral”), and 1 cm anterior to native (“anterior”) (Figure 2A-C). For the anterior position, a 1 cm thick plastic bone block was placed between the shingle and the tibia while maintaining its native position in the coronal plane. The lateral position was intended to represent the presurgical pathologic state (increased TT-TG), the native position a postsurgical medialized state, and the anterior position a postsurgical anteromedialized state. PF contact pressures were recorded using an electronic pressure sensor (sensor #5040, Tekscan, Boston, MA). Contact data was separated to the medial and lateral facets by identifying the median patellar ridge on the sensor. Within each facet, the sum of forces and center of pressure (weighted average of position of all acting forces within the facet relative to the median patellar ridge) was computed. Kinematics were recorded using a reflective marker motion capture system (Cortex, Motion Analysis Corporation, Santa Rosa, CA). Repeated measures ANOVA with post hoc Bonferroni analysis was used to determine differences in contact force and center of pressure location for each tubercle position. Statistical significance was defined as p<0.05. Results: There was a significant increase in the lateral facet, medial facet, and total patellar contact forces with lateral tubercle position compared to the anterior position (Figure 3). There was also a significant increase in medial facet and total patellar contact forces with the native tubercle position compared to the anterior position. There were no significant differences in lateral facet, medial facet, or total patellar contact forces when comparing the native and lateral tubercle positions. There was a trend toward an increased (lateralized) lateral facet center of pressure when comparing the lateral and anterior tubercle positions (Figure 4). Conclusions: Using a model capable of quantifying kinematics and contact mechanics for dysplastic trochleae and patellae, we demonstrated that an anterior tubercle position resulted in decreased patellar contact forces when compared to lateralized and native tubercle positions. These findings suggest that when an AMZ TTO is performed in the setting of an increased TT-TG distance and PF dysplasia, overall patellar contact forces are reduced. This may improve PF biomechanics and potentially decrease the likelihood of future PF OA. Similar findings were not observed for the native tubercle position, suggesting that anterorization is a critical consideration in improving PF biomechanics in this setting.

Dysplastic Patellofemoral Joints Lead to a Shift in Contact Forces: A 3D-Printed Cadaveric Model

Background: The distribution of contact forces across the dysplastic patellofemoral joint has not been adequately quantified because models cannot easily mimic the dysplasia of both the trochlea and the patella. Thus, the mechanical consequences of surgical treatments to correct dysplasia cannot be established. Purpose/Hypothesis: The objective of this study was to quantify the contact mechanics and kinematics of normal, mild, and severely dysplastic patellofemoral joints using synthetic mimics of the articulating surfaces on cadavers. We tested the hypothesis that severely dysplastic joints would result in significantly increased patellofemoral contact forces and abnormal kinematics. Study Design: Controlled laboratory study. Method: Patellofemoral dysplasia was simulated in 9 cadaveric knees by replacing the native patellar and trochlear surfaces with synthetic patellar and trochlear implants. For each knee, 3 synthetic surface geometries (normal, showing no signs of dysplasia; mild, exemplifying Dejour type A; and severe, exemplifying Dejour type B) were randomized for implantation and testing. Patellar kinematics and the sum of forces acting on the medial and lateral patellar facets were computed for each knee and for each condition at 10° increments from 0° to 70° of flexion. Results: A pronounced lateral shift in the weighted center of contact of the lateral facet occurred for severely dysplastic knees from 20° to 70° of flexion. Compared with normal geometries, lateral patellar facet forces exhibited a significant increase only with mild dysplasia from 50° to 70° of flexion and with severe dysplasia at 70° of flexion. No measurable differences in medial patellar facet mechanics or joint kinematics occurred. Conclusion: Our hypothesis was rejected: Severely dysplastic joints did not result in significantly increased patellofemoral contact forces and abnormal kinematics in our cadaveric simulation. Rather, severe dysplasia resulted in a pronounced lateral shift in contact forces across the lateral patellar facet, while changes in kinematics and the magnitude of contact forces were not significant. Clinical Relevance: Including dysplasia of both the patella and trochlea is required to fully capture the mechanics of this complex joint. The pronounced lateralization of contact force in severely dysplastic patellofemoral joints should be considered to avoid cartilage overload with surgical manipulation.

Patella Apex Influences Patellar Ligament Forces and Ratio

The relationship between three-dimensional shape and patellofemoral mechanics is complicated. The Wiberg patella classification is a method of distinguishing shape differences in the axial plane of the patella that can be used to connect shape differences to observed mechanics. This study uses a statistical shape model to relate the Wiberg patella classification to patella height and investigates its role in force distribution within the patellofemoral joint. The Wiberg Type I patella is shortest and has a more symmetrical medial and lateral facet while the Type III patella is longest with a larger lateral facet compared to medial. We generated patellofemoral morphologies from the statistical shape model and integrated them into a musculoskeletal model with a twelve degrees-of-freedom knee. We simulated an overground walking trial with these morphologies and recorded patellofemoral mechanics and ligament forces. An increase in patellar ligament force corresponded with an increase in patella height. Wiberg Type III patellas had a sharper patella apex which related to lower ratios of quadriceps tendon forces to patellar ligament forces. The change in pivot point of the patella affects the ratio of forces as well as the patellofemoral reaction force. This study provides a better understating of how patella morphology affects fundamental patella mechanics which may help identify at-risk populations for pathology development.

Patellar Shape is Associated With Femoral Trochlear Morphology in Individuals with Mature Skeletal Development

Background: As several studies have detected correlations between patellar and femoral trochlear development, this raises the question of whether patellar shape is associated with trochlear developmental outcomes.Methods: Patellar shape and femoral trochlear morphology were retrospectively analyzed in 240 subjects, of whom 80 each were classified as having Wiberg type I, II, and III patellae (groups A, B, and C, respectively). The sulcus angle (SA), lateral trochlea inclination angle (LTA), medial trochlear inclination angle (MTA), lateral facet length (LFL), medial facet length (MFL), lateral trochlear height (LTH), medial trochlear height (MTH), trochlea sulcus height (TH), and lateral-medial trochlear facet distance (TD) were analyzed as a means of evaluating trochlear morphology. Trochlear depth, trochlear condyle asymmetry, and trochlear facet asymmetry were additionally calculated, and differences in trochlear morphology and correlations between trochlear morphology and patellar shape were evaluated.Results: The femoral trochlear parameters of patients in group A differed significantly from those of patients in groups B and C. No significant differences between groups B and C were evident. Patellar shape was positively correlated with LTA, MTA, MFL, trochlear index trochlear condyle asymmetry, and trochlear facet asymmetry, and was negatively correlated with SA.Conclusions: These data indicated that patellar shape and trochlear morphology are related to one another. Relative to patients with Wiberg type II and III patellae, those with Wiberg type I patellae exhibited an increased trochlear inclination angle and a greater trochlear facet and condyle asymmetry, as well as a decreased SA.Trial registration: Retrospectively registered

Changes in the Contact Stress Distribution Pattern of the Patellofemoral Joint After Medial Open-Wedge High Tibial Osteotomy: An Evaluation Using Computed Tomography Osteoabsorptiometry

Background: Medial open-wedge high tibial osteotomy (OWHTO) theoretically causes distalization and lateralization of the tibial tuberosity and the patella. Purpose/Hypothesis: The purpose of the study was to identify any changes in the stress distribution of subchondral bone density across the patellofemoral (PF) joint before and after OWHTO through the use of computed tomography (CT) osteoabsorptiometry. We hypothesized that OWHTO would alter the distribution of contact stress in the PF joint. Study Design: Case series; Level of evidence, 4. Methods: A total of 17 patients (17 knees) who underwent OWHTO were enrolled in this study between September 2013 and September 2015. All patients underwent radiologic examination preoperatively and at 1 year postoperatively, and the distribution patterns of subchondral bone density through the articular surface of the femoral trochlea and patella were assessed preoperatively and >1 year postoperatively using CT osteoabsorptiometry. The quantitative analysis of the obtained mapping data focused on location of the high-density area (HDA) through the articular surface of the PF joint. The percentage of HDA at each divided region of the articular surface of the femoral trochlea and the patella was calculated. Results: In the radiologic evaluation, the Blackburne-Peel ratio was significantly reduced ( P < .001) after surgery, and the tilting angle of the patella was significantly decreased ( P < .001). On CT evaluation, the percentage of HDA in the lateral notch and lateral trochlea of the femur and in the medial portion of the lateral facet of the patella increased significantly after OWHTO surgery ( P ≤ .038). Conclusion: OWHTO significantly increased the stress distribution pattern of the lateral trochlea of the femur and the medial portion of the lateral facet of the patella. The procedure significantly lowered the patellar height and significantly decreased the patellar tilting angle after surgery.

THE RELATIONSHIP BETWEEN PATELLAR MORPHOLOGY AND LATERAL PATELLAR INSTABILITY

Background: Lateral patellar instability (LPI) is a substantial cause of morbidity in the pediatric population. Previously identified risk factors for LPI include trochlear dysplasia, a lateralized tibial tubercle, genu valgum, femoral anteversion, and external tibial torsion. Less is known regarding the relationship between patellar morphology and LPI. Purpose: The goal of this study is to determine whether there exists a relationship between patellar morphology and LPI. Methods: Magnetic resonance imaging (MRI) evaluation was performed for patients under 18 years of age with LPI and compared to a control group of MRIs of patients with anterior cruciate ligament (ACL) rupture. Using T2 axial MRI images, the lateral and medial facet angle of both the bone and cartilage of the patella was measured at three locations: the most proximal and distal aspects of the patella where the cartilage of the facets could be identified and the widest point of the patella. The width of the patella at each point was also recorded, resulting in 15 total data points per subject (5 at each of the three locations on the patella). Results were analyzed and compared between the instability group and the control group to determine any relationship between facet angle and LPI. Results: 196 MRIs were reviewed, 97 in the instability group and 96 in the control group. The LPI group was noted to have a less steep angle at the proximal medial patellar facet of both the bone (LPI 27.2° ± 9.3° ; control 32.7° ± 8.8°, p < 0.001) and cartilage (LPI 26.5° ± 8.8°, control 32.7° ± 8.4°, p < 0.001) as well as a less steep angle of the cartilage at the distal lateral facet (LPI 23.4° ± 7.2°, control 25.6° ± 6.6°, p = 0.033). No other differences were noted for the remaining 12 data points. Conclusion: The are very few differences in patellar morphology between patients with and without LPI. Patients were LPI have a less steep angle of the bone of the proximal medial facet, the cartilage of the proximal medial facet, and the cartilage of the distal lateral facet when compared to a control group. [Figure: see text]

Lateral facet growth of ice and snow – Part 1: Observations and applications to secondary habits

Often overlooked in studies of ice growth is how the crystal facets increase in area, that is, grow laterally. This paper reports on observations and applications of such lateral facet growth for vapor-grown ice in air. Using a new crystal-growth chamber, we observed air pockets forming at crystal corners when a sublimated crystal is regrown. This observation indicates that the lateral spreading of a face can, under some conditions, extend as a thin overhang over the adjoining region. We argue that this extension is driven by a flux of surface-mobile molecules across the face to the lateral-growth front. Following the pioneering work on this topic by Akira Yamashita, we call this flux “adjoining surface transport” (AST) and the extension overgrowth “protruding growth”. Further experiments revealed other types of pockets that are difficult to explain without invoking AST and protruding growth. We develop a simple model for lateral facet growth on a tabular crystal in air, finding that AST is required to explain observations of facet spreading. Applying the AST concept to observed ice and snow crystals, we argue that AST promotes facet spreading, causes protruding growth, and alters layer nucleation rates. In particular, depending on the conditions, combinations of lateral- and normal-growth processes can help explain presently inexplicable secondary features and habits such as air pockets, small circular centers in dendrites, hollow structure, multiple-capped columns, scrolls, sheath clusters, and trigonals. For dendrites and sheaths, AST may increase their maximum dimensions and round their tips. Although these applications presently lack quantitative detail, the overall body of evidence here demonstrates that any complete model of ice growth from the vapor should include such lateral-growth processes.

Does Greater Trochanter Decortication Affect Suture Anchor Pullout Strength in Abductor Tendon Repairs? A Biomechanical Study

Background: Greater trochanter decortication is frequently performed at the time of abductor tendon repair to theoretically increase healing potential. No previous studies have determined the effect that greater trochanter decortication has on the pullout strength of suture anchors. Hypothesis/Purpose: The purpose of this study is to determine whether greater trochanter decortication and bone mineral density affect suture anchor pullout strength in abductor tendon repair. The authors hypothesize that both will have a significant detrimental effect on suture anchor pullout strength. Study Design: Controlled laboratory study. Methods: Nineteen cadaveric proximal femurs with accompanying demographic data and computed tomography scans were skeletonized to expose the greater trochanter. Bone density measurements were acquired by converting Hounsfield units to T-score, based on a standardized volumetric sample in the intertrochanteric region of the femur. The gluteus medius insertion site on the lateral facet of the greater trochanter was evenly divided into 2 regions, anterior-distal and posterior-proximal, and each region was randomly assigned to receive either no decortication or 2 mm of bone decortication. A single biocomposite anchor was implanted in each region and initially tested with cyclic loading for 10 cycles at 0-50 N, 0-100 N, 0-150 N, and 0-200 N, followed by load to failure (LTF) tested at 1 mm/s. For each trial, the number of cycles endured, LTF, mechanism of failure, and stiffness were recorded. Results: Greater trochanters with no decortication and 2 mm of decortication survived a mean ± SD 35.1 ± 6.4 and 28.5 ± 10.6 cycles, respectively ( P < .01). Load to failure for nondecorticated specimens was 206.7 ± 75.0 N versus 152.3 ± 60.2 N for decorticated specimens ( P < .001). In a multivariate analysis, decortication and bone density were determinants in LTF ( P < .05). Conclusion: Decortication and decreased bone mineral density significantly decreased the pullout strength of suture anchors in the lateral facet of the greater trochanter. Clinical Relevance: Bone density should be considered when determining whether to perform greater trochanter decortication in abductor tendon repairs.

Increased Magnetic Resonance Imaging Signal of the Lateral Patellar Facet Cartilage: A Functional Marker for Patellar Instability?

Background: In the knee joint, predisposition for patellar instability can be assessed by an abnormal Insall-Salvati index, tibial tuberosity–trochlear groove (TTTG) distance, and abnormal shape of patella and trochlea. Given the complex anatomic features of the knee joint with varying positions of the patella during motion, the presence of a single or even a combination of these factors does not inevitably result in patellar instability. After trocheoplasty in patients with trochlear dysplasia, assessment of trochlear cartilage and subchondral bone is limited due to postoperative artifacts. Identification of presence of edema in the patellar cartilage may be helpful to identify patellar instability before and after surgery in these patients. Purpose: To determine whether increased signal intensity of the lateral patellar facet cartilage or measurements of abnormal patellofemoral articulation are associated with patellar instability before and after trochleoplasty. Study Design: Case series; Level of evidence, 4. Methods: Twenty-two patients with clinical diagnosis of patellar instability who underwent trochleoplasty, with magnetic resonance imaging (MRI) of the knee before and after surgery, were identified. The following observations and measurements were obtained in preoperative imaging: Insall-Salvati ratio, tibial tuberosity–trochlear groove (TTTG) distance, patellar shape (Wiberg), trochlear shape (Hepp), and edema in the lateral patellar facet cartilage. At 3 to 12 months after surgery, the presence or absence of edema in the cartilage of the lateral facet of the patella, the trochlear shape, and TTTG distance were reassessed. Wilcoxon matched-pairs signed rank test and Student t test were used. Interreader agreement was calculated as the Cohen κ or paired Student t test. Results: Increased cartilage signal was present in 20 patients before trochleoplasty and in 4 after trochleoplasty. Insall-Salvati ratio was greater than 1.20 in 20 patients. Patellar shape was greater than type 2 in 18 patients. Trochlear shape was greater than type 2 in 21 patients before and 7 after trochleoplasty. Mean TTTG distance was 14 mm before and 10 mm after surgery. When results before and after surgery were compared, a significant difference was found for cartilage signal, TTTG distance, and trochlear shape. Agreement for observations was moderate to substantial, and no significant differences were found for interreader agreement ( P > .05). Conclusion: Patellar cartilage at the lateral facet of the patella can be assessed after trochleoplasty despite postoperative artifacts in the trochlea. A decrease of patellar edema seems to be associated with improved femoropatellar articulation. Moreover, patellar edema may be used as a functional criterion of patellofemoral instability. This would provide additional information compared to morphologic criteria which just describe predisposing factors for femoropatellar instability.