Femoral head diameter has a major influence on stability and dislocation resistance of the hip joint after Total Hip Arthroplasty (THA). Dual Mobility (DM) implants can also reduce the risk of dislocation due the large diameter mobile liner which forms the femoroacetbular articulation. However, recent studies have shown that large head prostheses can directly impinge against native soft tissues, particularly the iliopsoas, leading to anterior hip pain. Dual mobility systems have emerged as a revision option in the treatment of failed metal on metal devices because of the high incidence of post revision instability secondary to abductor loss and need for capsulectomy. We hypothesized that an Anatomically Contoured Dual Mobility (ACDM) liner could provide joint stability while better accommodating the soft tissues surrounding the hip joint. The dislocation resistance of a 44 mm ACDM implant was compared to that of a 44 mm conventional DM liner. Both implants consisted of a 28 mm inner small diameter head and the liner was abducted to be in the worst case position for dislocation (Fig. 1). The ACDM liner was based on a 44 mm sphere with smaller radii used to contour the peripheral region below the equator of the liner. MSC Adams was used for dynamic simulations based on two previously described dislocation modes: (A) Posterior dislocation (at 90° hip flexion) with internal rotation of the hip and a posterosuperior directed joint force; (B) Posterior dislocation (starting at 90° flexion) with combined hip flexion and adduction and a posteromedial force direction (Fig. 2). Impingement-free motion (motion without neck impingement against the acetabular cup) and jump distance (head separation from acetabulum at dislocation) were measured for each implant. The acetabular cup was placed at 42.5° abduction and 19.7° anteversion, while the femoral component was anteverted by 9.75° based on published data.INTRODUCTION
METHODS
While kinematic abnormalities of contemporary TKA implants have been well established, a solution has not yet been achieved. We hypothesized that contemporary TKA implants are not compatible with normal soft-tissue function and normal knee motion. We propose a novel technique for reverse engineering advanced implant articular surfaces (biomimetic surface), by using accurate 3D kinematics of normal knees. This technique accounts for surgical placement of the implants, and allows design of tibial and femoral articular surfaces in conjunction. Magnetic resonance imaging was used to create 3D knee models of 40 normal subjects (24 male, 16 female, age 29.9 ± 9.7 years), and bi-planar fluoroscopy was used to capture 3D knee motion during a deep knee bend. These data were combined to create a 3D virtual representation of an average normal knee and its motion pathway. A TKA femoral component was mounted on the average knee, and moved through its normal kinematic pathway to carve out an articular surface from a tibial template (Fig. 1 and 2). The geometry of the resulting biomimetic tibia was compared to that of the native tibia, and a contemporary TKA tibial insert that uses the same femoral component.Introduction:
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Large diameter femoral heads have been used successfully to prevent dislocation after Total Hip Arthroplasty (THA). However, recent studies show that the peripheral region of contemporary femoral heads can directly impinge against the native soft-tissues, particularly the iliopsoas, leading to activity limiting anterior hip pain. This is because the spherical articular surface of contemporary prosthesis overhangs beyond that of the native anatomy (Fig. 1). The goal of this research was to develop an anatomically shaped, soft-tissue friendly large diameter femoral head that retains the benefits of contemporary implants. Various Anatomically Contoured femoral Head (ACH) designs were constructed, wherein the articular surface extending from the pole to a theta (θ) angle, matched that of contemporary implants (Fig. 2). However, the articular surface in the peripheral region was moved inward towards the femoral head center, thereby reducing material that could impinge on the soft-tissues (Fig. 1 and Fig. 2). Finite element analysis was used to determine the femoroacetabular contact area under peak in vivo loads during different activities. Dynamic simulations were used to determine jump distance prior to posterior dislocation under different dislocation modes. Published data was used to compare the implant articular geometry to native anatomy (Fig. 3). These analyses were used to optimize the soft-tissue relief, while retaining the load bearing contact area, and the dislocation resistance of conventional implants.Introduction:
Methods:
Dual Mobility (DM) hip implants have gained popularity for the treatment and preventions of instability. In DM implants a large diameter mobile insert matches the native femoral head size. However, studies have shown that the peripheral regions of such large diameter implants overhang beyond the native anatomy and can directly impinge against nearby soft tissues, especially the iliopsoas, leading to groin pain (Fig. 1). Soft-tissue impingement can also trap the mobile DM insert, leading to damage of its peripheral rim, which secures the small diameter inner head (Fig. 2). The goal of this research was to develop an anatomically contoured soft-tissue friendly DM insert. Various Anatomically Contoured Dual Mobility (ACDM) insert designs were constructed, wherein the outer articular surface extending from the pole to a theta (θ) angle, matched that of contemporary implants (Fig. 3). However, the articular surface in the peripheral region was moved inward towards the center, thereby reducing implant volume that could impinge on the soft tissue (Fig. 1 and Fig. 3). Finite element analyses were used to determine the insert-acetabular contact area under peak in vivo loads during different activities. Finite element analysis was also used to determine resistance to extraction of the inner head. Published data was used to compare the implant articular geometry to native anatomy. These analyses were used optimize the soft-tissue relief, while matching the load bearing contact area and the resistance to extraction of the inner head in contemporary implants.Introduction:
Methods:
Large diameter femoral heads provide increased range-of-motion and reduced dislocation rates compared to smaller diameter femoral heads. However, several recent studies have reported that contemporary large head prostheses can directly impinge against the local soft tissues leading to anterior hip pain. To address this we developed a novel Anatomically Contoured large diameter femoral Head (ACH) that maintains the profile of a large diameter femoral head over a hemispherical portion and then contours inward the distal profile of the head for soft-tissue relief. We hypothesized that the distal contouring of the ACH articular surface would not affect contact area. The impact of component placement, femoral head to acetabular liner radial clearance, and joint loading during different activities was investigated. A finite element model was used to assess the femoroacetabular contact area of a 36 mm diameter conventional head and a 36 mm ACH (Fig. 1). It included a rigid acetabular shell, plastically deformable UHMWPE acetabular liner, rigid femoral head and rigid femoral stem. The femoral stem was placed at 0°, 10° and 20° of anteversion. The acetabular shell and liner were placed in 20°, 40° and 60° of abduction and 0°, 20° and 40° of anteversion. The femoral head to acetabular liner radial clearances modeled were 0.06 mm, 0.13 mm and 0.5 mm. Three loading cases corresponding to peak in vivo loads during walking, chair sit and deep-knee bend were analyzed (Fig. 2). This allowed a range of component positions and maximum joint loads to be studied.Introduction
Methods
Dual mobility (DM) implants provide increased stability and range-of-motion through the use of a large diameter mobile liner articulating against an acetabular shell. However, recent studies have reported that such contemporary large head prostheses can directly impinge against the local soft tissues leading to anterior hip pain. To address this drawback, a novel Anatomically Contoured Dual Mobility (ACDM) liner was developed that maintains the outer spherical geometry over an approximately hemispherical portion and then contours inward the distal profile of the DM liner for soft-tissue relief. The extent of the inner profile encapsulating the small diameter head is increased to provide more coverage of the head and maintain the inner head pullout force. We hypothesized that the ACDM liner for soft-tissue relief would not affect retention of the small diameter inner head or liner-acetabular load-bearing contact area. A finite element model to evaluate head retention and contact mechanics was created with a rigid acetabular shell, a plastically deformable UHMWPE DM liner, a rigid femoral head and a rigid femoral stem. For the head retention analysis, the extent of head coverage (Fig. 1) was optimized to match the inner head pullout force of a conventional DM liner. Contact mechanics of a conventional DM and ACDM liner were analyzed at the maximum joint load of three activities: gait, deep-knee bend and chair sit. One set of simulations was completed with the mobile liner and head axes aligned and another with the axes mal-aligned so that the mobile liner rim was adjacent to the femoral stem neck and the potential area of contact was away from the mobile liner apex. This allowed a broader range of potential contact to be assessed including what was determined to be a worst-case alignment.Introduction
Methods
Contemporary Posterior Cruciate Ligament (PCL) retaining TKA implants (CR) are associated with well-known kinematic deficits, such as absence of medial pivot motion, paradoxical anterior femoral sliding, and posterior femoral subluxation at full extension. The hypothesis of this study was that a biomimetic implant, reverse engineered by using healthy knee kinematics to carve the tibial articular surface, could restore normal kinematic patterns of the knee. Kinematics of the biomimetic CR and two contemporary CR implants (A, B) were evaluated during simulated deep knee bend and chair-sit in LifeModeler KneeSIM™ software. Anteroposterior motion of the medial and lateral femoral condyle centers was measured relative to a tibial origin. The implants were mounted on an average knee model created from magnetic resonance imaging (MRI) of 40 healthy knees. The medial and lateral collateral ligaments, posterior cruciate ligament, quadriceps mechanism, and the overall capsular tension were modeled. The soft-tissue insertions were obtained from the average knee model, and the mechanical properties were obtained from literature. In vivo knee kinematics of healthy subjects from published literature was used for reference.Introduction:
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Acetabular component orientation has been linked to hip stability as well as bearing mechanics such as wear. Previous studies have demonstrated wide variations of cup placement in hip arthroplasty using conventional implantation techniques which rely upon either anatomic landmarks or the use of commercial positioning guides. Enabling technologies such as navigation have been used to improve precision and accuracy. Newer technologies such as robotic guidance have been postulated to further improve accuracy. The goal of our study was to evaluate the clinical reproducibility of a consecutive series of haptically guided THR. 119 patients at 4 centers were enrolled. All patients had preoperative CT scans for the purpose of planning cup placement in lateral opening and version using proprietary software (Mako, Ft. Lauderdale, FL). All procedures were performed using a posterolateral approach. Following bone registration, acetabular preparation and component position is performed using haptic guidance. Final implant postion is ascertained by obtaining 5 points about the rim of the acetabular component and recorded. At 6 weeks, all patients had AP and cross-table lateral radiographs which were then analyzed for cup abduction and anteversion using the Hip Analysis Suite software. The goal was to determine the variability between desired preoperative plan, intraoperative measurement and postoperative results.Introduction:
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Acetabular cup position is an important factor in successful total hip arthroplasty (THA). Optimal cup placement requires surgeons to possess an accurate perception of pelvic orientation during cup impaction, however, varying pelvic anatomy and limited visual cues in the surgical field may interfere with this process. The purpose of this study was to evaluate the utility of an inertial measurement unit (IMU) in monitoring pelvic position during THA. Ten patients scheduled to undergo THA were IRB-approved and consented by four surgeons. A small IMU was placed over the patient's sacrum pre-operatively and zeroed in standing position. Pelvic orientation data was streamed and captured wirelessly throughout the procedure. Surgeons were blinded to all data throughout the study period. Prior to cup impaction, the surgeon indicated his intended cup abduction angle and the degree to which the cup impactor was manipulated to compensate for perceived AP pelvic tilt. The degree of pelvic tilt as determined by the IMU (angle β) was then recorded (Figure 1). AP-pelvis radiographs were measured in Martell Hip Analysis Suite post-operatively to calculate the cup abduction angle, which was then compared to the surgeon's intended abduction angle to determine surgeon accuracy. To predict the final cup abduction angle, the degree of pelvic tilt recorded by the IMU (angle β) was subtracted from the abduction angle of the cup impactor (angle α) that was positioned using the OR table as a reference (Figure 1). This value was then compared to the measured post-operative cup abduction angle in order to assess the accuracy of the IMU in measuring pelvic tilt. Surgeon accuracy and IMU accuracy were compared to determine if the IMU was more or less effective than surgeon perception at determining pelvic tilt.Introduction:
Materials & Methods:
Femoral head diameter has a major influence on stability and dislocation resistance after Total Hip Arthroplasty (THA). Although routine use of large heads is common, several recent studies have shown that contemporary large head prostheses can directly impinge against native soft tissues, particularly the iliopsoas which wraps around the femoral head, leading to refractory anterior hip pain. To address this, we developed a novel Anatomically Contoured large diameter femoral Head (ACH). We hypothesized that anatomical contouring of the ACH implant for soft tissue relief would not compromise dislocation resistance, and the ACH implant would provide increased stability compared to small heads. In this study the dislocation resistance of a 36 mm ACH was compared to that of 28 mm and 36 mm contemporary heads. The ACH implant was based on a 36 mm sphere with smaller radii used to contour the peripheral region below the equator of the head. MSC Adams was used for dynamic simulations based on two previously described dislocation modes: (A) Posterior dislocation (at 90° hip flexion) with internal rotation of the hip and a posterosuperior directed joint force; (B) posterior dislocation (starting at 90° flexion) with combined hip flexion and adduction and a posteromedial force direction (Fig. 1). Impingement-free motion (motion without neck impingement against the acetabular liner) and jump distance (head separation from acetabulum prior to dislocation) were measured to evaluate the dislocation risk of each implant. The acetabular cup was placed at 42.5° abduction and 19.7° anteversion, while the femoral component was anteverted by 9.75° based on published data.INTRODUCTION
METHODS
In total hip arthroplasty, the positioning of the acetabular cup, in particular, has been shown to play an important role in the survivorship of the prosthetic joint. The commonly accepted “safe zone” extends from 5–30° of anteversion to 30–50° of inclination. However, several studies have utilized a more restrictive safe zone of 5–25° of anteversion and 30–45° of inclination, a modification of the Lewinnek zone. Many attempts have been made to develop a more reliable method of positioning the acetabular component. Robotic-assisted surgery is one such method. The purpose of this study was to compare the resulting position of the acetabular component after robotic-assisted surgery with the intraoperative robotic data to determine if improved accuracy can be achieved with the robotic-assisted method. One hundred and nineteen patients received THA, at four different medical centers in the United States, using a haptic robotic arm. Pre-operative CT scans were obtained for all patients and used during the planning of the procedure, at which point the proposed component size and positioning was determined. Preparation of the acetabular bone bed, as well as impaction of the acetabular component itself, was performed using the robotic device. Using an AP Pelvis and Cross-Table Lateral radiograph, each patient's resulting acetabular inclination and version was measured using the Hip Analysis Suite software. The component position retrieved from the robot was compared to the measured values from the radiographs. The positioning data was compared to two safe zones described above.Introduction
Methods
