Functional Anatomy of the Hip Joint.

BACKGROUND: The functional anatomy of the osteoarthritic hip joint in the sagittal plane has not been defined. The purpose of this study was to define the functional anatomy of the hip using clinical and radiographic analyses. METHODS: 320 hips had preoperative standing and sitting lateral spine-pelvis-hip X-rays. Radiographic pelvic measurements were pelvic incidence (PI) and sacral slope (SS), and hip measurements were anteinclination (AI) and pelvic femoral angle (PFA). Pelvic tilt (PT) was calculated as PI-SS. A triangle model was created from the clinical data that illustrates the functional motion of the hip during postural changes from standing to sitting. RESULTS: Pelvic motion was coordinated with hip motion, even with spinopelvic imbalance and stiffness. Pelvic motion (ΔSS) varied for all 5 types of imbalance, but pelvic motion (ΔSS) and acetabular motion (ΔAI) changed with a 1:1 ratio and inversely with femoral motion (ΔPFA) with a 1:1 ratio. The triangle model showed similar results with ΔSS, ΔPT, and ΔAI changing in a 1:1:1 ratio, and femur motion inversely changing with a 1:1 ratio. CONCLUSION: The functional anatomy of the hip joint can be visually illustrated using a triangle model. Pelvic angles SS, PT, and AI change in unison, whereas femoral motion (ΔPFA) changes inversely with pelvic motion (ΔSS) in a 1:1 ratio. This coordinated mobility explains the limitations of the Lewinnek safe zone, which include only the acetabulum.

Is Combined Anteversion Equally Affected by Acetabular Cup and Femoral Stem Anteversion?

BACKGROUND: To create a safe zone, an understanding of the combined femoral and acetabular mating during hip motion is required. We investigated the position of the femoral head inside the acetabular liner during simulated hip motion. We hypothesized that cup and stem anteversions do not equally affect hip motion and combined hip anteversion. METHODS: Hip implant motion was simulated in standing, sitting, sit-to-stand, bending forward, squatting, and pivoting positions using the MATLAB software. A line passing through the center of the stem neck and the center of the prosthetic head exits at the polar axis (PA) of the prosthetic head. When the prosthetic head and liner are parallel, the PA faces the center of the liner (PA position = 0, 0). By simulating hip motion in 1-degree increments, the maximum distance of the PA from the liner center and the direction of its movement were measured (polar coordination system). RESULTS: The effect of modifying cup and stem anteversion on the direction and distance of the PA's change inside the acetabular liner was different. Stem anteversion influenced the PA position inside the liner more than cup anteversion during sitting, sit-to-stand, squatting, and bending forward (P = .0001). This effect was evident even when comparing stems with different neck angles (P = .0001). CONCLUSION: Cup anteversion, stem anteversion, and stem neck-shaft angle affected the PA position inside the liner and combined anteversion in different ways. Thus, focusing on cup orientation alone when assessing hip motion during different daily activities is inadequate.

The Effect of Functional Pelvic Tilt on the Three-Dimensional Acetabular Cup Orientation in Total Hip Arthroplasty Dislocations.

BACKGROUND: Anterior and posterior pelvic tilt appears to play a role in total hip arthroplasty (THA) stability. When changing from the standing to the sitting position, the pelvis typically rotates posteriorly while the hips flex and this affects the femoro-acetabular positions. This case-control study compares changes in 3-D acetabular cup orientation during functional pelvic tilt between posterior THA dislocations vs stable THAs. METHODS: Standing and sitting 3-D cup orientation was compared between fifteen posterior dislocations vs 233 prospectively followed stable THAs. 3-D cup orientation was calculated using previously validated trigonometric algorithms on biplanar radiographs. Those algorithms combine the angles in the three anatomical planes (coronal inclination, transverse version, and sagittal ante-inclination) in the standing position with the change in sagittal pelvic tilt from standing to sitting to calculate the 3-D orientation in the sitting position. RESULTS: The standing cup orientation of the dislocated THAs was only characterized by a lower coronal inclination (P = .039). Compared with the controls, from standing to sitting, they showed less posterior pelvic tilt (P < .001). This led to a significant lower coronal inclination (P < .001) and sagittal ante-inclination (P < .001) in the sitting position but similar transverse version (P = .366). CONCLUSIONS: Comparing posterior THA dislocations to stable THAs, there is a lower increase of all three orientation angles from standing to sitting. This leads to a decreased sitting coronal inclination and sagittal ante-inclination which may lead to an increased risk of impingement ensued by THA instability. By contrast, the transverse version was not significantly different in both positions. This confirms the importance of biplanar data on functional cup orientation. LEVEL OF EVIDENCE: Diagnostic, Level III.

Spinopelvic Motion and Impingement in Total Hip Arthroplasty.

The stability of a total hip arthroplasty relies on proper positioning of the acetabular cup. Recent research has shown that this cup position is more dynamic than previously thought. The 3-dimensional orientation of the acetabular cup changes when the pelvis tilts anteriorly or posteriorly. These changes in pelvic tilt are directly related to the biomechanics of the lumbosacral junction. In normal physiology, the lumbar spine straightens with sitting and becomes more lordotic with standing. This directly translates to posterior or anterior pelvic tilt due to the rigid sacroiliac attachments. During sitting, increased posterior pelvic tilt opens the acetabulum to accommodate flexion and internal rotation of the hip. This helps prevent anterior impingement and posterior hip dislocation. During standing, anterior pelvic tilt increases superior coverage of the acetabulum. This helps prevent posterior impingement and anterior hip dislocations. When lumbosacral motion becomes pathologic, spinopelvic motion changes and acetabular cup orientation is affected. In cases of decreased lumbosacral motion, patients rely on greater hip motion to reach standing or sitting positions. This can cause pathologic impingement. In addition, traditional safe zones for cup position may not apply in the presence of pathologic spinopelvic motion. This article discusses the normal physiology of spinopelvic motion, the patterns of pathologic change, and the clinical implications therein.

Functional Safe Zone Is Superior to the Lewinnek Safe Zone for Total Hip Arthroplasty: Why the Lewinnek Safe Zone Is Not Always Predictive of Stability.

BACKGROUND: The Lewinnek "safe zone" is not always predictive of stability after total hip arthroplasty (THA). Recent studies have focused on functional hip motion as observed on lateral spine-pelvis-hip x-rays. The purpose of this study was to assess the correlation between the Lewinnek safe zone and the functional safe zone based on hip and pelvic motion in the sagittal plane. METHODS: Three hundred twenty hips (291 patients) underwent primary THA using computer navigation. Two hundred ninety-six of these hips (92.5%) were within the Lewinnek safe zone as determined by inclination of 40° ± 10° and anteversion of 15° ± 10°. All patients had preoperative and postoperative standing and sitting lateral spinopelvic x-rays. The combined sagittal index (CSI), a combination of sagittal acetabular and femoral position, was measured for each patient and used to assess the functional safe zone. Data analysis was performed to identify hips in the Lewinnek safe zone inside and outside the sagittal functional safe zone. Predictive factors for hips outside the functional safe zone were identified. RESULTS: Of the 296 hips within the Lewinnek safe zone, 254 (85.8%) were also in the functional safe zone. Forty-two patients were outside the functional safe zone based on CSI; 19 had an increased standing CSI and 23 had a decreased sitting CSI, all were considered at risk for dislocation. Predictive factors for falling outside the functional safe zone were increased femoral mobility (P < .001, r = 0.632), decreased spinopelvic mobility (P < .001, r = 0.455), and pelvic incidence (P < .001, r = 0.400). CONCLUSION: In this study, 14.2% of hips within the Lewinnek safe zone were outside the functional safe zone, identifying a potential reason hips dislocate despite having "normal" cup angles. The best predictor for falling outside the functional safe zone, both preoperatively and postoperatively, was femoral mobility, not the sagittal cup position (ie, cup anteinclination). LEVEL OF EVIDENCE: Level III, retrospective review.

The Opioid Crisis and the Orthopedic Surgeon.

Opioid use and abuse has become a national crisis in the United States. Many opioid abusers become addicted through an initial course of legal, physician-prescribed medications. Consequently, there has been increased pressure on medical care providers to be better stewards of these medications. In orthopedic surgery and total joint arthroplasty, pain control after surgery is critical for restoring mobility and maintaining patient satisfaction in the early postoperative period. Before the opioid misuse epidemic, orthopedic surgeons were frequently influenced to "treat pain with pain medications." Long-acting opioids, such as OxyContin were used commonly. In the past decade, there has been a paradigm shift in favor of multimodal pain control with limited opioid use. This review will discuss 4 major topics. First, we will describe the pressures on orthopedic surgeons to prescribe narcotic pain medications. We will then discuss the major and minor complications and side effects associated with these prescriptions. Second, we will review how these factors motivated the development of alternative pain management strategies and a multimodal approach. Third, we will look at perioperative interventions that can reduce postoperative opioid consumption, including wound injections and peripheral nerve blocks, which have shown superb clinical results. Finally, we will recommend an evidence-based program that avoids parenteral narcotics and facilitates rapid discharge home without readmissions for pain-related complaints.

The Current Knowledge on Spinopelvic Mobility.

Recent studies may suggest that our conventional knowledge of risk factors for dislocation may need rethinking. Previous studies have demonstrated a large majority of total hip arthroplasty instability with acetabular cups implanted in safe zones. Recently discovered spinopelvic motion is a coordinated biomechanical relationship among acetabular anteversion, pelvic tilt, and lumbar lordosis. Classification includes normal, hypermobile, stiff, stuck standing, stuck sitting, and fused. Normal spinopelvic motion from standing to sitting occurs with hip flexion, posterior sacral tilt, and decreased lumbar lordosis to accommodate a flexed femur and prevent impingement and dislocation. Acetabular cup implantation ideally is adapted based on spinopelvic interactions. This may lower the rate of impingement and subsequent dislocation. These new biomechanical interactions may provide a better understanding of the safe zones of anteversion and inclination.

Outpatient Arthroplasty is Here Now.

Substantial advances have been made in arthroplasty to minimize surgical trauma and maximize perioperative pain control, which has enabled patients to regain mobility within hours of surgical intervention and be safely discharged to home the same day. Surgeons should understand the indications and contraindications for the safe performance of outpatient arthroplasty in a hospital and ambulatory surgical center setting as well as know how to optimize, medically manage, prepare, and rehabilitate patients. To undertake outpatient arthroplasty, surgeons must be knowledgeable in multimodal anesthesia techniques, effective venous thromboembolism prophylaxis, blood management, and wound management. In addition, surgeons must learn the subtle nuances of specialized surgical techniques that lend themselves to outpatient arthroplasty, including partial knee, muscle-sparing total hip, less invasive total knee, and total shoulder techniques.

Posterior Mini-Incision With Primary Total Hip Arthroplasty: A Nine to Ten Year Follow Up Study.

The question has been raised as to whether small incision surgery will compromise long term results of total hip arthroplasty. We report nine to ten years' outcome with posterior mini-incision. Radiographs were measured for component position, polyethylene wear, fixation, and osteolysis. Sixty-two of the original 86 patients (76 of 100 hips) were alive and available for study with 17 patients deceased (with known results) and seven (8%) lost to follow-up. The result was rated as excellent in 70 of 75 remaining hips (93%). Eighty-nine of 93 hips (96%) with known results had the original implants. Radiographically, wear was a mean 0.015 ± 0.009 mm/year, and no hip had impending failure. There were four revisions, 2 for dislocation, 1 for fracture, and 1 for loose cup.