title: Biomechanical effects of bone-implant fitness and screw breakage on the stability and stress performance of the nonstemmed hip system.
abstract: BACKGROUND
  Some nonstemmed hip systems have been developed to avoid stress shielding and aseptic loosening, which are major drawbacks of stemmed hip arthroplasty. Without the stem, the cup over the femoral head can be stabilized by anatomic fitness of the cup interior and mechanical fixation of the auxiliary screws.
  
  
  METHODS
  Using finite-element method, neck-shaped systems with two bone-cup fitness situations and four types of screw breakages are systematically investigated to evaluate their biomechanical effects on construct performances. The construct stresses and interfacial micromotion were chosen for comparison between two bone-cup fitness situations and four types of screw breakages.
  
  
  RESULTS
  The screw breakage deteriorates the stresses of the mating screw and the neck cup and loosens the bone-cup interfaces. The breakages of central and locking screws decrease the bone stress by about 43.2% and 12.7%, respectively. This indicates that the central screw is a more effective load-bearer for the superimposed cup than the locking screw. As compared with the fitting cup, the stress of cup and the bone stresses of the unfitting cup obviously increase. This demonstrates that the load-transferring path at the cup bottom is important in directly relieving the prosthetic stresses.
  
  
  CONCLUSIONS
  Any screw design inducing stress concentration should be validated to avoid screw breakage. Comparatively, surgical unfitness has a more significant effect on the construct performance than does the screw breakage. Even for custom-made cups, cautious preparation of the neck resection is still necessary to ensure intimate bone-cup contact.
authors.affiliation: Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan.
authors.forename: Chun-Ming
authors.initials: CM
authors.lastname: Chen
authors.affiliation: Department of Orthopaedic Surgery, Mackay Memorial Hospital, Hsinchu, Taiwan.
authors.forename: Chih-Ting
authors.initials: CT
authors.lastname: Cheng
authors.affiliation: Department of Orthopedic Surgery, Mindong Hospital, Fujian Medical University, Fuan, China.
authors.forename: Cheng-Shou
authors.initials: CS
authors.lastname: Lin
authors.affiliation: Graduate Institute of Biomedical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. Electronic address: orthodent.cax@gmail.com.
authors.forename: Shang-Chih
authors.initials: SC
authors.lastname: Lin
authors.affiliation: BoneCare Orthopedic Centers, Han-Chiung Clinics, Taipei, Taiwan.
authors.forename: Chien-Chung
authors.initials: CC
authors.lastname: Chiang
authors.affiliation: Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan.
authors.forename: Chu-An
authors.initials: CA
authors.lastname: Luo
authors.affiliation: Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan.
authors.forename: Ching-Shiow
authors.initials: CS
authors.lastname: Tseng
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation*; D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws; D019544:Equipment Failure Analysis; D005270:Femur Head; D005272:Femur Neck; D020342:Finite Element Analysis; D006622:Hip Prosthesis; D006801:Humans; D016268:Internal Fixators; D008297:Male; D009119:Muscle Contraction; D011474:Prosthesis Design / Q000379:methods*; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D016474:Weight-Bearing; D055815:Young Adult
keywords: Finite-element; Hip nonstemmed; Neck cup; Screw breakage; Stress-shielding