Spontaneous Healing of Avascular Necrosis of the Femoral Head in Sickle Cell Disease
Authors: Itzep Nelda , Jadhav Siddharth , Kanne Celeste K , Sheehan Vivien A 
Published: October 2019

Abstract
Background. Avascular necrosis (AVN) is a serious complication of sickle cell disease (SCD) that can lead to significant morbidity including chronic pain and physical impairment with treatment depending on the patient's pain severity and functional limitation. Patient age, hemoglobin levels, and abnormal rheology, particularly high blood viscosity, or thickness, and percentage of dense red blood cells, defined as cells >1.11 mg/mL have been identified as risk factors for the development of AVN, but their contributions to the subsequent clinical course has not been described.
The natural history of AVN has been described as chronic and progressive without significant surgical or physical therapy based intervention. However, this description was established before widespread MRI use in diagnosis, or hydroxyurea (HU). A new evaluation of the natural history of AVN is needed.
Objective: Review the natural history of AVN at our institution in patients for whom repeat imaging is available, and determine if patient demographics, treatment type, and laboratory data, including rheological values, predict of clinical course.
Methods. We identified all patients with a diagnosis of AVN with diagnostic and follow-up imaging studies (pelvic X-rays or MRI of the hip), receiving care at Texas Children's Hospital Hematology Center 2006 to 2018, and collected demographic, treatment, rheological and clinical laboratory data. Patient radiographic images at diagnosis and follow up were reviewed and scored by a board certified pediatric radiologist using the Steinberg criteria. Laboratory values for each patient were collected as available from time of diagnosis through to the date of follow up imaging. Longitudinal patient blood samples were collected for rheological measurements under an IRB approved protocol. Whole blood viscosity was measured on a cone and plate viscometer (DV3T Rheometer, AMETEK Brookfield, Middleboro, MA, USA) at 45 and 225 s-1 at 37oC within 4 hours of sample collection in an EDTA vacutainer tube. Percent dense red blood cells (%DRBCs) were measured by an ADVIA 120 Hematology System (Siemens Medical Solutions USA, Inc., Malvern, PA, USA). Statistical analysis comparing groups with and without radiographic improvement was performed using a two-tailed Student's t-test.
Results. 16 patients had repeat imaging, allowing for assessment of radiographic improvement (Table1). All were diagnosed via X ray or MRI of the pelvis. Five of 16 patients had resolution or significant radiographic improvement of AVN without orthopedic intervention or prolonged physical therapy, while 11 patients had stable or worsening disease. In the group with radiographic improvement, the mean age was 9.9 years (SD=2.5) and 14.6 years (SD=2.2) in the group without radiographic improvement (p= 0.002). Five patients underwent core decompression with bone marrow aspirate concentrate injection; one progressed to complete femoral head collapse while the rest had stable or worsening disease. Mean hemoglobin (Hb), fetal hemoglobin (HbF), percent DRBCs, whole blood viscosity, and extent of AVN at diagnosis by Steinberg criteria were not statistically different between the patients that showed improvement and those that did not.
Conclusions. Our results challenge the paradigm of untreated AVN in SCD as an inexorably progressive disease. Here we present 5 cases of significant radiographic improvement of AVN without surgical or significant physical therapy interventions. Conversely, 5 AVN cases treated with core decompression remained stable or worsened. Given our small cohort, we were unable to establish laboratory or rheological predictors of spontaneous resolution. However, patients in the radiographic improvement group were younger than patients with progressive disease. HU has shown promise in improving vascular complications in SCD patients however more research is needed to delineate its role as a modifier of AVN natural history as it was not correlated with the improved group in our study (all our study patients were on HU either prior to diagnosis or started during the follow up period). Given the findings that significant radiographic improvement is possible with minimal or no intervention, providers should closely weigh the risks and benefits of observation versus surgical intervention, particularly in young patients with SCD.

Disclosures
No relevant conflicts of interest to declare.