title: Quantification and correlation of hip capsular volume to demographic and radiographic predictors.
abstract: OBJECTIVE
  The purpose of this study was to develop a novel method to quantify hip capsular volume in patients undergoing hip arthroscopic surgery, utilizing magnetic resonance arthrogram (MRA) and to determine whether there are demographic or radiological factors that are associated with capsular volume.
  
  
  METHODS
  A retrospective review was performed from 2006 to 2014 of consecutive patients who presented with hip pain and received a hip MRA and plain radiographs. All patients were suspected of soft tissue injury due to underlying femoroacetabular impingement (FAI). A novel technique using Osirix MD for the quantification of capsular and femoral head volumes was described.
  
  
  RESULTS
  Ninety-seven patients met the study criteria and were included for analysis. The average total capsular volume (including the femoral head) measured 79.89 ± 20.35 cm(3), average femoral head volume 46.68 ± 12.32 cm(3), and average true capsular volume measured 33.20 ± 12.58 cm(3). Average total capsular:femoral head volume ratio was 1.74 ± 0.27. Significant differences were seen between genders for total capsular volume (P < 0.01), femoral head volume (P < 0.01), and true capsular volume (P < 0.01). Total capsular volume:femoral head ratio was greater for females, but was not statistically significant (n.s.). Femoral head volume significantly correlated with alpha angle (P < 0.01), height (P < 0.01), BMI (P < 0.01), BMI (P = 0.02), and age (P < 0.01). Total capsular volume significantly correlated with height (P < 0.01), BMI (P = 0.01), and age (P < 0.01). Age was also correlated with true capsular volume (P = 0.011). No significant differences in capsular volumes were found between normal and abnormal radiographic measurements.
  
  
  CONCLUSIONS
  The current study describes a reproducible radiographic measurement for hip capsule volumes from MRAs. Only gender was predictive of total capsular volume, femoral head volume, and true capsular volume. There were no macroscopic anatomical differences evident on MRA. This method showed good intra- and inter-observer reliability and can aid in future research regarding hip capsule volumes. This novel technique may potentially allow clinicians a readily available and reliable method to detect large and redundant capsules, a possible predisposition for hip micro-instability.
  
  
  METHODS
  Retrospective case series, Level IV.
authors.affiliation: Division of Sports Medicine, Department of Orthopedic Surgery, Hip Preservation Center, Rush University Medical Center, Rush Medical College of Rush University, 1611 W. Harrison St, Suite 300, Chicago, IL, 60612, USA.
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Frank
authors.affiliation: Division of Sports Medicine, Department of Orthopedic Surgery, Hip Preservation Center, Rush University Medical Center, Rush Medical College of Rush University, 1611 W. Harrison St, Suite 300, Chicago, IL, 60612, USA.
authors.forename: Simon
authors.initials: S
authors.lastname: Lee
authors.affiliation: Holy Cross Orthopedic Institute, 5597 N. Dixie Highway, Oakland Park, FL, 33334, USA.
authors.forename: Frank M
authors.initials: FM
authors.lastname: McCormick
authors.affiliation: Division of Sports Medicine, Department of Orthopedic Surgery, Hip Preservation Center, Rush University Medical Center, Rush Medical College of Rush University, 1611 W. Harrison St, Suite 300, Chicago, IL, 60612, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Jordan
authors.affiliation: Division of Sports Medicine, Department of Orthopedic Surgery, Hip Preservation Center, Rush University Medical Center, Rush Medical College of Rush University, 1611 W. Harrison St, Suite 300, Chicago, IL, 60612, USA.
authors.forename: Bryce
authors.initials: B
authors.lastname: Austell
authors.affiliation: Division of Sports Medicine, Department of Orthopedic Surgery, Hip Preservation Center, Rush University Medical Center, Rush Medical College of Rush University, 1611 W. Harrison St, Suite 300, Chicago, IL, 60612, USA.
authors.forename: William
authors.initials: W
authors.lastname: Slikker
authors.affiliation: Case Western University, 11100 Euclid Ave, Cleveland, OH, 44106, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Salata
authors.affiliation: Division of Sports Medicine, Department of Orthopedic Surgery, Hip Preservation Center, Rush University Medical Center, Rush Medical College of Rush University, 1611 W. Harrison St, Suite 300, Chicago, IL, 60612, USA. Shane.nho@rushortho.com.
authors.forename: Shane J
authors.initials: SJ
authors.lastname: Nho
mesh_terms: D000328:Adult; D000367:Age Factors; D001182:Arthroscopy; D005260:Female; D006621:Hip Joint / Q000000981:diagnostic imaging* / Q000601:surgery; D006801:Humans; D017746:Joint Capsule / Q000000981:diagnostic imaging* / Q000601:surgery; D008279:Magnetic Resonance Imaging; D008297:Male; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012737:Sex Factors
keywords: Hip; Hip arthroscopy; Hip capsule; Hip instability; Radiological evaluation