ABML2011

O1-6

2011年11月3日-5日 東京（芝浦工業大学）

アクティブおよびパッシブ力を発生させることが可能な骨折モデルの開発
Active and passive fracture reduction force simulations using a musculoskeletal model developed withpneumatic McKibben actuators○ ソフィアサイドシク（東京大）鄭常賢（東京大）

大西五三男（東京大）小林英津子（東京大）別所雅彦（東京大）

大橋暁（東京大）

佐久間一郎（東京大）Sofiah SYED SHIKH, University of Tokyo
Sanghyun JOUNG, University of Tokyo
Etsuko KOBAYASHI, University of Tokyo
Satoru OHASHI, University of Tokyo
Isao OHNISHI, University of Tokyo
Masahiko BESSHO, University of Tokyo
Ichiro SAKUMA, University of Tokyo
Abstract: A p h y s i c a l m o d e l o f a f r a c t u r e d f e m u r a n d i t s s u r r o u n d i n g s o f t t i s s u e i s d e v e l o p e d i n o r d e r
to simulate active and passive muscle forces during a robot assisted fracture reduction procedure.
I n s u c h p r o c e d u r e s , b e s i d e s a i m i n g f o r a c c u r a c y i n r e d u c t i o n , s m a l l e r m a g n i t u d e r e d u c t i o n fo r c e sa b s o r b e d b y t h e s u r r o u n d i n g s o f t t i s s u e wi l l r e d u c e t h e r i s k o f f u r t h e r i n j u r y t o t h e p a t i e n t a n di n c r e a s e t h e c h a n c e s o f r e c o v e r y. T h e d e v e l o p e d mo d e l u s e s p n e u m a t i c M c K i b b e n a c t u a t o r s t or e p r e s e n t t h e a c t i v e a n d p a s s i v e m u s c l e s . S i m u l a t i o n o f t h e d e v e l o p e d mo d e l c a n b e c o mp a r e d t of o r c e s i m u l a t i o n v a l u e s f r o m a p r e v i o u s l y d e v e l o p e d H i l l m u s c l e mo d e l . T h e s e fo r c e s i m u l a t i o n swi l l b e f u n d a m e n t a l i n c o n s t r u c t i n g r e d u c t i o n p a t h s wi t h t h e l e a s t r e d u c t i o n fo r c e fo r t h e a s s i s t i v e
f r a c t u r e r e d u c t i o n r o b o t . I n a d d i t i o n , t h e d e v e l o p e d r e a l i s t i c p h y s i c a l m o d e l s wi l l b e b e n e fi c i a l a sa t o o l fo r fr a c t u r e r e d u c t i o n t r a i n i n g .
Key Words: Fracture reduction force, musculoskeletal model, pneumatic muscle
1. Introduction
Fracture reduction describes the medical procedure to restore
a fractured bone to its original alignment. This procedure iscarried out before a fracture fixator can be inserted to fix thepatient’s fracture condition. Reductions are commonly
performed manually by the surgeons but drawbacks of manualreduction include there being no quantitative measure forapplied forces, the excessive radiation exposure and it being
physically demanding for surgeons. A robot assisted fracture
reduction system is an alternative solution to theseshortcomings as it minimizes human error, provides higher
fracture reduction accuracy and allows a quantifiable measureof the applied force as demonstrated by our developed system(1).
One major concern with the use of surgical robots in medical
procedures is patient safety. The success of a robot assisted
fracture reduction procedure is commonly measured by thenonexistence of mechanical failures during the surgicalprocedure and the percentage of accurate restoration of theoriginal bone alignment. However, one area that is often
overlooked is the condition of the soft tissues surrounding thebone involved in the reduction procedure.
In order to understand the soft tissue response during thisprocedure, a physical model of the fractured femur and its
surrounding soft tissue is being developed. Diaphyseal or longbone fracture of the femur is chosen to be mimicked as it has
been reported to be one of the more commonly occurring
fractures. Criteria for the model include matching the anatomyof the fractured limb and mimicking the response of thesurrounding tissue during the fracture reduction procedure.

With the general goal of increasing the accuracy and safety ofa robot assisted fracture reduction procedure, this research aims
to develop physical models in order to realistically simulate
forces in the surrounding soft tissue during fracture reduction.
In addition, the physical models can be used as fracture
reduction training tools, where they can be utilized as aplatform for practice prior to performing the actual procedureon patients.
2. Prototype
2-1 Physical musculoskeletal model
A physical model of the lower limb is developed in order tomimic the muscle force response. In particular, the model aims
to mimic the conditions of the bone and its surrounding soft
tissue for a diaphyseal fracture of the femur.
McKibben actuators, which are a type of pneumatic artificial
muscles reported to have performance similar to that of
biological muscles, were used to mimic the response of theactual muscles. Previous research has shown that for actuator
lengths less than resting length, the actuator provides a firstorder approximation of biological muscles(2). The McKibben
actuator used in this study is made up of a rubber tube
surrounded by a tough plastic weave, developed by the Shadow
Robot Company Ltd. (3). Two different actuator lengths, 210 mm
and 290 mm, were used to build this model. These actuators
have the ability to contract when the inner rubber tube part of
the actuator is inflated with pressurized compressed air andwould otherwise be in extension.
For this preliminary model, one muscle out of each anterior,
O1-6-1posterior, medial and lateral compartments were replicated.
These representative muscles are expected to exhibit a greater
response when simulated as compared to their counterparts intheir respective groups, based on their biological functions. The
muscles that were replicated are the rectus femoris (anterior),biceps femoris (posterior), adductor longus (medial) andiliotibial tract (lateral). In order to closely mimic thebiomechanical properties of biological muscles, the insertionand origin points of the aforementioned muscles were closelymimicked. In addition, biological muscle resting lengths werealso observed.

Fig. 1the fracture reduction procedure on the musculoskeletal modeland on an actual patient.
3. Experimental Results
Since the preliminary simulations were carried out without
activating the pneumatic muscles, force resistance during thefracture reduction trials came from the passive properties of thepneumatic muscle. Force-length experiments were carried out inorder to understand the passive response of the pneumaticmuscles (Figure 3).

Physical musculoskeletal model of a diaphyseal fracture
femur

Fig. 3Force-length relationship of the pneumatic muscles
with length 210 mm (medium) and 290 mm (large)

2-2 Fracture reduction simulations
Manual fracture reduction simulations were performed using
the developed musculoskeletal model. The initial position of theproximal and distal fractured bone pieces were set for each trial
and an orthopedic surgeon performed the fracture reduction
procedure repeatedly for 7 trials. The model was covered during
the trials and the surgeon performed the fracture reduction
procedure guided by the previously developed navigation
system(1). Reflective markers were placed on the proximal anddistal bone to track the bone positions and a force sensor wasattached to the handle which was connected to the surgeon’shand. This enabled a direct recording of the total amount offorce applied by the surgeon to the model. Each trial was
considered complete and successful when the surgeon managed
to match the distal and proximal bone pieces as close as
possible to its original position after a series of traction androtation. The duration for each trial was also recorded.

In terms of fracture reduction force, the force applied on themusculoskeletal model showed a common progression between
trials and also that with a manual reduction. Namely, thereduction force started with an increase in force where the distal
fragment of the bone undergoes traction, then is rotated and thefinal stage shows little fluctuation in force where small
adjustments is made to finalize the bone position (Figure 4).

Fig. 4

Fig. 2

Resultant reduction force for fracture reduction carriedout on the bone-soft tissue model

Qualitatively, the developed musculoskeletal model has beenfound to produce similar resistance when compared to an actual
biological environment. Additional resistance and components
of soft tissue has been suggested to be added in order to further
closely mimic the biomechanics of the bone-soft tissue
environment, this includes the neuromuscular system.

Initial (left) and end (right) position of the distal andproximal fractured bone pieces

The pneumatic muscles were not activated during these initialtrials. Muscles were not also pre-stretched, so the resistance felt
by the surgeon was solely due to the pneumatic muscles’
inherent properties. Feedbacks were also acquired from thesurgeon regarding the differences and similarities of performing
O1-6-2

4. Discussion
The main aim of fracture reduction procedures is toaccurately reposition the fractured bone pieces into its originalposition. With the development of robot assisted fracture
reduction systems, the accuracy and safety of the procedure isgreatly improved as compared to current manual procedures.
This study aims to further increase the level of safety of fracture
reduction procedures performed with the robot assisted system
by focusing on minimizing the fracture reduction force applied
to the bone-soft tissue interaction. By minimizing the forces atthe soft tissue surrounding the fracture site, additional soft
tissue injuries can be avoided which leads to a better prognosis
for the patient. Developing a realistic musculoskeletal model is
expected help in better understanding the bone-soft tissue
interaction during the fracture reduction procedure.
The current model introduces a basic bone-soft tissue modelwith passive force feedback. Qualitative and quantitative
validations have to be carried out in order to accurately replicate
the biomechanical properties of the biological system. Currently,
qualitative validations come from surgeon feedbacks obtained
during fracture reduction simulation trials. These feedbacks areimportant in improving the design of the model and thisfeedback-modification cycle is expected to continue throughout
the design process. In order to further closely replicate
biological bone-muscles interactions, a control system iscurrently being developed to mimic the force response ofindividual muscles. These force responses will then be
implemented to control the pneumatic muscles active responsein the musculoskeletal model. In addition, simulation of force
values from individual muscle will be carried out to allowdetermination of fracture reduction forces from specificmuscles.
The musculoskeletal model is also developed to be used astraining tools for practicing surgeons and medical students alike.
Since the model mimics the bone-soft tissue environment, users
will be able to practice performing the fracture reduction
procedure prior to performing it on an actual patient. This
allows the users to familiarize themselves with the actual
surrounding and learn to maneuver the system in order tominimize any possible errors during the actual procedure, hence
contributing to the overall goal of increasing patient safety inrobot assisted fracture reduction systems.
References(1) S. Joung, H. Kamon, H. Liao, J. Iwaki, T. Nakazawa, M.
Mitsuishi, Y. Nakajima, T. Koyama, N. Sugano and Y. Maeda,“A Robot Assisted Hip Fracture Reduction with a Navigation
System,” Med Image Comp Assist Interv, vol. 11(Pt. 2), pp.
501- -508, 2008
(2) G. Klute, J. Czerniecki, and B. Hannaford, “McKibben
Artificial Muscles: Pneumatic Actuators with BiomechanicalIntelligence,” Proc. of IEEE/ASME Int. Conf. on AdvancedIntelligent Mechatronics, pp. 221-226, 1999.
(3) Shadow Robot Company Ltd, www.shadow.org.uk

O1-6-3