Background. In vivo fluoroscopic studies have proven that femoral head sliding and separation from within the acetabular cup during gait frequently occur for subjects implanted with a total hip arthroplasty. It is hypothesized that these atypical kinematic patterns are due to component malalignments that yield uncharacteristically higher forces on the hip joint that are not present in the native hip. This in vivo joint instability can lead to edge loading, increased stresses, and premature wear on the acetabular component. Objective. The objective of this study was to use forward solution mathematical modeling to theoretically analyze the causes and effects of hip joint instability and edge loading during both swing and stance phase of gait. Methods. The model used for this study simulates the quadriceps muscles, hamstring muscles, gluteus muscles, iliopsoas group, tensor fasciae latae, and an adductor muscle group. Other soft tissues include the patellar ligament and the ischiofemoral, iliofemoral, and pubofemoral hip capsular ligaments. The model was previously validated using telemetric implants and fluoroscopic results from existing implant designs. The model was used to simulate theoretical surgeries where various surgical alignments were implemented and to determine the hip joint stability. Parameters of interest in this study are joint instability and femoral head sliding within the acetabular cup, along with contact area, contact forces, contact stresses, and ligament tension. Results. During swing phase, it was determined that femoral head pistoning is caused by hip capsule laxity resulting from improperly positioned components and reduced joint tension. At the point of maximum velocity of the foot (approximately halfway through), the momentum of the lower leg becomes too great for a lax capsule to properly constrain the hip, leading to the femoral component pistoning outwards. This pistoning motion, leading to separation, is coupled with a decrease in contact area and an impulse-like spike in contact stress (Figure 1). During stance phase, it was determined that femoral head sliding within the acetabular cup is caused by the proprioceptive notion that the human hip wants to rotate about its native, anatomical center. Thus, component shifting yields abnormal forces and torques on the joint, leading to the femoral component sliding within the cup. This phenomenon of sliding yields acetabular edge-loading on the supero-lateral aspect of the cup (Figure 2). It is also clear that joint sliding yields a decreased contact area, in this case over half of the stable contact area, corresponding to a predicted increase in contact stress, in this case over double (Figure 2). Discussion. From our current analysis, the causes and effects of hip joint instability are clearly demonstrated. The increased stress that accompanies the pistoning/impulse loading scenarios during swing phase and the supero-lateral edge-loading scenarios during stance phase provide clear explanations for premature component wear on the cup, and thus the importance of proper alignment of the THA components is essential for a maximum THA lifetime. For any figures or tables, please contact authors directly

Background. Extensive research has previously been conducted analyzing the biomechanical effects of rotational changes (i.e. version and inclination) of the acetabular cup. Many sources, citing diverse dislocation statistics, encourage surgeons to strive for various “safe zones” during the THA operation. However, minimal research has been conducted, especially under in vivo conditions, to assess the consequences of cup translational shifting (i.e. offsets, medial and superior reaming, etc.). While it is often the practice to medialize the acetabular cup intraoperatively, there is still a lack of information regarding the biomechanical consequences of such cup medializations and medial/superior malpositionings. Objective. Therefore, the objective of this study is to use a validated forward solution mathematical model to vary cup positioning in both the medial and superior directions to assess simulated in vivo kinematics. Methods. The model used for this study has been validated with telemetric data and incorporates numerous muscles and ligaments. The model is parametrically derived and allows the user to simulate a theoretical THA surgery and to assess the outcomes of proper positioning as well as malpositioning of the cup. Parameters of interest in this study are component positions, joint instability and sliding, and contact area. Results. An intraoperative representation of the pelvis and cup was assessed (Figure 1), with a green star showing the native anatomical center, the red circle showing the acetabular cup center, and the arrow representing the reaming direction. During swing phase, it was determined that unaccounted for acetabular cup shifting of 5–10 mm leads to capsular ligament laxity coupled with an increase in hip joint instability. Two swing phase scenarios were assessed, one simulating adequate capsular tension and therefore a uniform contact patch and the other simulating inadequate capsule tension and therefore femoral component pistoning with a smaller contact patch (Figure 2). During stance phase, it was determined that acetabular cup shifting of 5–10 mm in the medial and/or superior directions yields an increase in hip joint instability. Two stance phase scenarios were simulated, one yielding no hip separation and therefore a uniform, centralized contact patch, and the other yielding ∼1.5 mm of hip separation and therefore a non-uniform, supero-lateral edge loading patch (Figure 3). Cup orientation does not appear to directly cause hip instability, but it will either lessen or exacerbate the instability, depending on the specific scenario. The results in this study did reveal that overly-inclined cups will yield less stability in the lateral direction, and overly-anteverted cups will yield less stability in the anterior direction. Discussion. In general, instability during stance phase comes in the form of femoral head sliding and edge loading, and instability during swing phase comes in the form of femoral head pistoning. This study's analyses did reveal that proper alignment of the acetabular cup is required for ideal clinical results. The results from this study dictate that proper translational alignment of the cup as well as rotational alignment is necessary for patient stability and proper hip mechanics. For any figures or tables, please contact authors directly

Total hip arthroplasty (THA) is one of the most successful surgery. However, patients' expectations have increased over the last two decades in regards to hip function after joint replacement, the patients assume to return their daily and sport activities without major limitations. This presentation will examine the effect of surgical approaches and implant designs as well as rehabilitation protocol on the clinical and biomechanical outcomes after THA. The new implant designs for THA aim to improve joint function whereas the surgical approaches intend to reduce muscle damage to regain muscle strength. One important determinant measured from gait analysis is the hip abduction moment as the abductors play a key role in stabilizing the pelvis in the frontal plane, particularly in phases of transition, such as the single leg stance in walking or stair climbing. This showed that muscle strength needs to be preserved. To minimize the risk of hip joint instability, a strong focus of implant development has been carried out. To illustrate this important concept within the context of gait analysis, I will present two studies that examine the influence of surgical approach and biomechanical reconstruction; and the second, is a prospective RCT comparing a dual mobility implant to a standard total hip replacement

Introduction. The purpose of this study was to demonstrate postoperative improvement and high satisfaction rates after a surgical approach that includes arthroscopic labral repair only, in patients with borderline dysplasia, without instability. Methods. Between September 2009 and December 2011, patients less than 50 years old who underwent hip arthroscopy for symptomatic intra-articular hip disorders, with a lateral center-edge (CE) angle between 20 and 25, were included in this study. Patients with Tönnis grade 2 or greater, hip joint space narrowing, severe hip dysplasia (CE〈20), hip joint instability and Legg-Calve-Perthes disease were excluded. Patient-reported outcome scores, including the modified Harris Hip Score (mHHS), Western Ontario and McMaster Universities Arthritis index (WOMAC), and visual analog scale (VAS) for pain were obtained in all patients preoperatively and at 1, 2, and 3 years postoperatively. Results. A total of 36 patients met the criteria to be included in the study. Of these, 32 (88.8%) patients were available for follow-up. There was a significant improvement in mHHS from 67.19±7.66 to 82.69±6.95 (P<0.001), and WOMAC score from 58.90±5.77 to 77.90±6.38 (P<0.001). There was a significant improvement in VAS scores from 5.8 to 2.9 (P<0.001). There was a significant improvement in range of motion, flexion from 108.44±7.77 to 115.31±6.08 (P<0.001) and external rotation from 29.06±5.74 to 33.13±4.88 (P<0.001). Conclusions. The current study demonstrates favorable results in borderline dysplasia hip without instability at minimum 3-year follow-up for an arthroscopic approach that includes labral repair. Labrum is the main key-stone structure, which should be preserved in borderline dysplasia hip for functional and pain improvement. Also the prognosis of treatment is probably forecasted more by the nature of stability and the intra-articular pathology rather than simply the presence or absence of radiographic finding of borderline dysplasia

Purpose. The purpose of this study was to demonstrate postoperative improvement and high satisfaction rates after a surgical approach that includes arthroscopic labral repair only, in patients with borderline dysplasia, without instability. Methods. Between September 2009 and December 2011, patients less than 50 years old who underwent hip arthroscopy for symptomatic intra-articular hip disorders, with a lateral center-edge (CE) angle between 20 and 25, were included in this study. Patients with Tönnis grade 2 or greater, hip joint space narrowing, severe hip dysplasia, hip joint instability and Legg-Calve-Perthes disease were excluded. Patient-reported outcome scores, including the modified Harris Hip Score (mHHS), Western Ontario and McMaster Universities Arthritis index (WOMAC), and visual analog scale (VAS) for pain were obtained in all patients preoperatively and at 1, 2, and 3 years postoperatively. Results. A total of 36 patients met the criteria to be included in the study. Of these, 32 (88.8%) patients were available for follow-up. There was a significant improvement in mHHS from 67.19 ± 7.66 to 82.69 ± 6.95 (P<0.05), WOMAC score from 58.90 ± 5.77 to 77.90 ± 6.38 (P<0.05), and VAS scores from 5.8 ± 0.88 to 2.9 ± 0.62 (P<0.05). There was a also improvement in range of motion, flexion from 108.44 ± 7.77 to 115.31 ± 6.08 (P<0.05) and external rotation from 29.06 ± 5.74 to 33.13 ± 4.88 (P<0.05). Conclusions. The current study demonstrates favorable results in borderline dysplasia hip without instability at minimum 3-year follow-up for an arthroscopic approach that includes labral repair. Labrum is the main key-stone structure, which should be preserved in borderline dysplasia hip for functional and pain improvement

Introduction:. The higher resisting torque against dislocation and the large range of motion due to the enlarged effective head diameter substantiate the use of eccentric dual-mobility cups in case of total hip joint instability [1,2]. As a result of force-dependent self-centering mechanism, an increased movement of the intermediate-component can be expected whose effect on wear propagation is unknown so far. Currently available hip joint simulators are only able to vary the load by the absolute value and not by the direction of resulting force. Therefore, the uniaxial force transmission may lead to a unique and stable alignment of the intermediate-component during testing. The purpose of this numerical study was to evaluate relative movements of the intermediate-component during daily life activities with respect to wear propagation. Method:. The numerical analysis was based on a standard dual-mobility system consisting of a polished metallic cup, a UHMWPE intermediate-component (40 mm outer diameter) with an eccentric offset of 2 mm and a 28 mm ceramic femoral head [Fig. 1]. The relative motion of the intermediate-component was affected by the geometrically generated self-centering torque (T. C. ) and the friction torque for inner (T. Fi. ) and outer (T. Fo. ) articulation around the centre of rotation Z. 1. [Fig. 2]. In order to consider lubrication conditions the lambda ratio was estimated for different daily life activities [3], including the calculation of composite roughness and minimum film thickness for a ball-on-plate configuration. The friction torque was related to the product of load (FRes), lever arm and the coefficients of friction which were determined using the equation proposed by [4]. Depending on load FRes and load orientation angle βthe equilibrium of moments around Z. 1. was established. Results:. Due to surface roughness and calculated minimum film thickness (0.1 to 0.62 μm), boundary lubrication was determined for both inner and outer articulation. The average coefficient of friction was 0.01825 for inner and 0.0751 for outer bearing. Torque patterns were obtained for different load orientation angles [Fig. 3]. In case of an angle β between 15° and 45° the friction torque dominated compared to the self-centering mechanism which resulted in a fixed intermediate-component position. Increasing the orientation angle β above 45° lead to a superiority of the self-centering effect as can be found for varus positions. Discussion:. In case of daily life activities the load vector FRes varies within the range of 30° in the frontal plane [3]. Therefore, the angle β ranges below 30° which leads to a fixed position of the intermediate-component. Consequently, the self-centering mechanism seems to have no negative influence on the wear propagation of dual-mobility cup systems. The presented numerical analysis permits a variation of implant-specific dimensions, but is limited to polyethylene wear couples

We reported a case of the acetabular depression fracture in conjunction with a central fracture dislocation of the hip that was treated with a unique surgical technique. CASE REPORT:. A 76-year-old man suffered a left acetabular fracture with severe left hip joint pain and walking disability. Acetabular fracture was not apparent on the initial radiographs including anteroposterior and oblique views of the pelvis. However, computed tomography (CT) scanning showed displaced acetabular depression fracture (a third fracture fragment) in the center of the weight-bearing area with fracture of the ilium and spontaneous reposition of central dislocation of the hip (Fig. 1, 2). It seemed that this fracture fragment created incongruity of the acetabular articular surface and the potential for hip joint instability. Therefore, the patient was treated with open reduction and internal fixation. SURGICAL TECHNIQUE:. To perform the procedure, the patient was placed in the lateral decubitus position. A direct lateral approach to the hip was used for exposure. The vastus lateralis was released 1 cm distal from its origin, trochanteric osteotomy was done by the Gigli saw. To observe the hip articular surface and to identify the fracture fragment, the femoral head was posterior dislocated with excision of teres ligamentum after T-shaped capsulotomy. The depressed fragment in the acetabulum was identified under direct vision but could not be reduced. Therefore, the outer cortex of the ilium was fenestrated in a size of 2 × 2 cm so that a 1-cm-wide levator was inserted to the depressed fragment at 2 cm proximal from the hip articular surface through the fenestrated window (Fig. 3). Subsequently, the displaced bone fragment was pushed down by using the levator to the adequate articular joint level. The fragment was stabilized with packed cancellous bone graft harvested from the osteotomized greater trochanter. The removed outer cortex of the ilium from fenestrated site was repositioned and fixed by a reconstruction plate and screws. The osteotomized greater trochanter was reattached and fixed with two cannulated cancellous hip screws. RESULTS:. At 9-month follow-up, he was pain-free and continued to function well without the use of external supports. The acetabular depression fracture was completely reduced and healed in the CT scanning evaluation. The patient had no signs of posttraumatic osteoarthritis in radiographs. DISCUSSION and CONCLUSION:. In acetabular fracture dislocations of the hip joint, the precise pathological anatomy is not easily demonstrated by routine radiographs with classification of acetabular fractures. In our case, however, details of acetabular fracture were not well visible on conventional radiographs. It has been shown that computed tomography is useful method in precise evaluation of the fracture type with bone damage and integrity of joint configuration. Concerning approach to the fracture fragment which existed in the center of the weight bearing area of acetabulum, we performed to fenestrate on the intact bony cortex of the ilium just proximal to the fracture site. It was convenient and useful to gain good reduction of the central acetabular depression fracture, although there was no report on such a ‘fenestration’ method

Aim: The purpose of this study is to determine the outcome in patients with acetabular fractures treated either conservatively or surgically. Method: From 1990–2000, we treated 152 patients with 158 acetabular fractures. 63 patients were treated nonoperatively and 95 operatively. According to Tile classification there were 70 type A, 52 type B, 36 type C fractures. Mean follow up was 90 months (23–151 months). Indications for surgery were fracture displacement of more than 2mm, hip joint instability, intrarticular fragments and ipsilateral femoral fracture. Surgical approaches used included the kocher-Langenbeck and the triradiate approach. Follow up consisted of radiological examination and functional assessment using Merle d’ Aubigne score. Results: 53 conservatively treated patients followed up. 39 (73.6%) had excellent and good results and 14 (26.4%) fair and poor results due to excessive fracture comminution, severe osteoporosis, or they were too sick to be operated on. 83 operatively treated patients with 85 fractures were followed-up. Anatomic reduction was achieved in 57 fractures, satisfactory in 18 and poor in 10 fractures. Functional outcome was excellent or good in 60 (72.3%) patients and fair and poor in 23 (27,7%).The complications were 3 wound infections, 4 cases of femoral head osteonecrosis, 3 cases of secondary loss of reduction and 5 cases of significant ectopic ossification. Conclusion: The outcome of these difficult fractures depends on restoration of hip joint congruity and stability and correlates closely to radiographic result