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Prototype Development of a Hand-Held Steerable
Tool for Hip Arthroscopy
Lindsey Paul, Timothy Chant, Ross Crawford, Jonathan Roberts, Liao Wu*
Australian Centre for Robotic Vision
Science and Engineering Faculty
Queensland University of Technology
Brisbane, Australia
Email: *liao.wu@qut.edu.au/dr.liao.wu@ieee.org
Abstract—Instruments used in hip arthroscopy are designed
to navigate to a target within the hip joint capsule and actuate
repairs using various tools. Traditional arthroscopy instruments
feature rigid shafts with no ability to adjust the curvature of the
arm or reorient the approach vector of the tool within the joint
capsule. To overcome these limitations, this paper has proposed a
novel robotized hand-held instrument for hip arthroscopy based
on a tendon-driven continuum manipulator segment and pivot
joint in series, with arm parameters based on the anatomical
dimensions of the joint capsule. An intuitive control system
is implemented to assist in surgeons’ operations. Preliminary
evaluations were made based on the prototype and workspace
reachability demonstrating the feasibility of the design.
I. INTRODUCTION
Arthroscopy of the hip is a form of minimally invasive
surgery (MIS), most commonly used to treat symptomatic
labral tears, femoroacetabular impingement (FAI), loose bod-
ies, synovitis, chondral defects and degenerative conditions
of the hip [1]. Typical procedures involve the use of an
arthroscope and rigid tools with varying end-effectors, such
as punches, scissors and probes to interact with the joint.
In this study, the tools used to actuate repairs, specifically
the rigid arm used to manipulate the position of the tool-
point, will be investigated. Typical arthroscopic tools consist
of a straight rigid tube, with a fixed tool at the distal end and
mechanical actuation of the tool controlled at the proximal end
of the device, which does not allow the operator to change the
approach vector of the tool point.
To overcome this drawback, a tendon-driven continuum
manipulator (TCM) segment is used to curve around the
femoral head, with a pivot joint in series to further manipulate
the approach angle of the tool, as illustrated in Fig. 1. These
two segments form 3 degrees of freedom (DOF) in the distal
15 mm of the arm, and by using an on-board gyroscope to
stabilize the base of the pivot joint, an additional DOF can be
added via manual rotation of the device.
A. Related Work
Hip arthroscopy has become increasingly popular, with
studies in the USA demonstrating a 365% increase in the
This work was supported by the Vice-Chancellor’s Research Fellowship
from Queensland University of Technology under Grant 322450-0096/08
awarded to Dr. Liao Wu.
Fig. 1. Illustration of the proposed arthroscopy device entering a distended
hip joint capsule through the standard anterior portal.
number of procedures performed between 2004 and 2009 and
a 250% increase between 2007 and 2011 [2].
The most prevalent arthroscopic instruments in current use
feature a straight or pre-curved rigid tube, between 2.7 mm and
3.4 mm in diameter, with a tendon operated tool mounted on
the distal 10 mm of the device. Although these instruments add
curvature to direct them through the cavity, most arthroscopic
tools cannot maneuverer in such small and tortuous spaces. To
overcome this difficulty in similar medical applications, nu-
merous works have been published on increasing tip dexterity
through modern methods of actuation, with methods typically
falling into two categories, extrinsic or intrinsic [3].
The EndoWrist R
created for da Vinci Surgical System
(Intuitive Surgical, U.S.A) and the telerobotic system for MIS
of the throat in [4], demonstrate the ability of extrinsic TCM
segments to increase tool dexterity, with both featuring 6 DOF
achieved by two TCM bending segments per arm, with diam-
eters of 5 mm and 4.2 mm respectively. The FlexDex TM [5]
and the flexible Cardioscope developed in [6] demonstrate the
adaptation of TCM for hand-held devices to create joints with
2 DOF, using mechanically controlled systems, with FlexDex
TM achieving a bend of 90 deg with a diameter of 8 mm and
the cardioscope bending 180 deg with an arm diameter of 6.5
mm. In [7], an example of a complex control system for hand-
held devices is demonstrated, designing a mechatronic tool for
computer assisted arthroscopy. This design features a 4 mm
diameter, 25 mm long TCM segment which provides 2 DOF
at the tip.
Concentric tube robots (CTRs), have been used to add 3
DOF to standard 3.8 mm diameter neuroendoscopes in [8],
and adopted as the mechanism for robotic tools in intraocular
[9] and transnasal surgery [10]. Like TCM, this mechnism
also reduces the outer diameter of the device at the expense
of control system weight and sizing.
Shape memory alloys (SMAs) have shown promise at
overcoming the control-sizing constraints of TCM and CTR,
with [11] achieving a maximum bending angle of 90 deg in
the distal 29 mm of a 5 mm diameter arm. Research in [12]
has also been done to combine SMA with a DC Micromotor
in series to achieve a 6 DOF surgical manipulator with a
spherical wrist and tool diameter of 5 mm. Although SMA
is significantly more compact than its counterparts, the low
stiffness, high activation voltage, and ineffectual heat removal
associated with this actuation method, lead to significant issues
in the implementation of SMA.
Devices allowing for the manipulation of the tool approach
angle are not common in related work, with no examples found
specifically for hip arthroscopy.
B. Contributions
This paper proposes the novel application of cable-actuated
arms in arthroscopy of the hip. The arm design features
two joints in series, designed to curve between the femoral
head and the acetabulum to the desired area, and allow for
manipulation of the tool point approach vector. The advantages
include: increased field of operation, having a design suitable
for a low-cost disposable arm, and decreasing the risk of dam-
age to cartilage and surrounding tissue by adding additional
dexterity. This arm design has been implemented in a hand-
held servo-actuated device for preliminary testing, but future
work will require a system with greater stability and accuracy
in actuation.
II. DESIGN
The designed prototype of the arthroscopic tool is shown
in Fig. 2. It comprises of: an arm with 3 DOF at the tip,
a channel for tool attachment through the arm, an actuation
system, housing, a control module, control circuits, a 12.6V
Li-ion battery (not shown in the figure), a handle with two
2-axis joysticks (not shown in the figure), and a USB port for
serial output. The total size of the arm is 3.4 mm x 3.4 mm x
11.2 mm, with a total weight of 30 g, and the total size of the
Fig. 2. Prototype of the proposed hand-held hip arthroscopy tool.
control system and housing is 70 mm x 75 mm x 155 mm,
with a total weight of 290 g.
A. Arm Design
For hip arthroscopy procedures, the end-effector needs to
be able to access and interact with the ligaments surrounding
the joint and all areas of the socket, including loose bodies
within the joint capsule. As the capsule is contained between
the femoral head and acetabulum, this can be used as the
bounds of the workspace. When a traction force is applied
to the hip joint, and the capsule is distended with saline, the
joint typically distracts 15 mm to 30 mm, with a joint capsule
volume of 3·10−5m3to 4·10−5m3[13]. With an average
femoral head diameter measurement of 47.3 mm [14] and an
average acetabulum diameter of 53.7 mm [15], the workspace
can be approximated as the gap between two spheres with the
given average diameters of the acetabulum and femoral head,
separated at the origins by 11.8 mm.
To achieve the desired movement, it was decided that two
joints were needed, with one to have 2 DOF of movement
and curve around the femoral head, and one to manipulate
the angle of the end-effector in that space. A 3-cable TCM
segment is suitable for the mid joint due to its maneuverability
and scalable dimensions. But due to the restricting nature of
the workspace, the bend radius of a TCM segment would not
be suitable for the distal joint. Therefore, a 2-tendon 1 DOF
pivot joint was selected to manipulate the tool approach, and
by applying manual rotation at the base of the device, 2 DOF
can be achieved, resulting in a total of 4 DOF in the device.
Design specifications for end-effector attachment were
based on dimensions suitable for the attachment of a standard
rolling shutter camera. Therefore, a 0.9 mm hollow was used
through the middle of the pivot joint and through the backbone
of the TCM joint, with a 2 mm slot at the tip of the pivot joint
to attach the end-effector. This also allows for the attachment
of standard rigid tools as they typically feature cables of less
than 0.9mm diameter to actuate their end-effectors, such as
graspers or punches.
Trocars used for tool insertion limit the diameter of the
arm to 4.5 mm, considering this as well as the confines of
the workspace, a maximum arm diameter of 3.4 mm was
chosen. Also, given that the pivot joint is designed for a
(a) (b)
Fig. 3. Design of proposed arthroscopy arm with 90 deg bend at the TCM
segment and no base rotation. (a) Side view. (b) Front view.
maximum rotation of 90 deg in either direction, the length
of the tool attached measured from the middle of the pivot
axis combined with the radius of the arm must not exceed the
15 mm distraction mentioned above.
As increased length decreases stability, a TCM length of 10
mm was chosen. Due to the limited applied force needed by
arthroscopes; to achieve a maximum bending radius of 90 deg
in a length of 10 mm, the TCM backbone was constructed
from PVC tubing with an ID of 1 mm and an OD of 2 mm.
To attach the TCM actuation cables to the end of the arc, a
rigid section of 3.6 mm was added between the TCM segment
and pivot joint. These arm specifications have been illustrated
in Fig. 3.
Construction of this design was achieved by manually
constructing each wire separation disc from PVC and attaching
it to the backbone with cyanoacrylate. Stainless steel wire
was used for actuation of the arm and the pivot joint was
constructed from sheet aluminum.
B. Control Design
As mentioned previously, the arm is made up of; a 3-cable
actuated TCM segment, a 2-cable actuated pivot joint and
an optional 1-cable actuated end effector, totaling 6 control
cables. Each cable’s displacement is independently controlled
by a servo motor as shown in Fig. 4a. To convert the rotational
movement to the required linear displacement each servo
motor is fitted with a 20 mm long arm as shown in Fig. 4b. A
consequence of this method is the displacement of the cables
relative to arm angle is not linear due to the circular movement
of the lever tip however this is corrected for in the firmware
using basic trigonometric functions.
Each servo has a maximum travel of ±40◦, with positional
resolution of 1024, this is sufficient for a prototype however
given the requirements for hip-arthroscopy higher accuracy
linear actuators will be used for future revisions. The maxi-
mum displacement was experimentally verified to be ±9mm
while resolution and repeatability were 1 mm.
To prevent damage to the arm, limits were placed in the
firmware as to how far the servos can displace the cables
in addition to a simple current monitoring circuit to infer
(a) Overview.
(b) TCM Joint Group.
Fig. 4. Design of the actuation system.
J1x-
R
B1
B2
J1y-
J2y+
J2y-
J2x+
J2x-
J1x+
Buttons
Joysticks
J1y+
Fig. 5. The proposed control interface based on joysticks and buttons.
approximate torque, powering down servos and sounding a
buzzer under over-torque conditions.
The device is based around an Atmel ATmega1280 MCU
(Micro Controller Unit), which accepts inputs from a combina-
tion of joysticks and buttons shown in Fig. 5, and actuates the
servos accordingly. Inputs from the first joystick control the
curvature, κ, and the rotation, φ, of the TCM segment, and the
second joystick controls the rotation, α, of the pivot joint and
the control of the tool attached. The peripheries shown in Fig.
5 allow the operator to manually control the arm, and the on-
board IMU (Inertial Measurement Unit) can be implemented to
allow for mid-joint position stabilization given manual rotation
at the base. This will cause an apparent additional DOF at
the pivot hinge, allowing the end-effector to rotate about the
central axis of the distal segment.
During operation of the device, serial data is output from
the USB port for diagnostics or data logging during operation.
The device can be powered from an isolated 12 V DC power
supply or a 12.6 V 25 Wh Li-ion rechargeable battery for
untethered operation.
Fig. 6. Block diagram of micro controller unit (MCU) peripheries.
C. Firmware Design
Simple firmware was implemented, with a core structure
consisting of two threads scheduled by two hardware timers.
The first thread running at approximately 0.5 Hz handles
miscellaneous tasks such as overload detection, status LEDs,
monitoring voltages and sending serial data packets. The other,
repeating at 10 Hz handles the electromechanical system, such
a periphery inputs, simple calculations and servo control.
The two joysticks shown in Fig. 5 alter arm parameters, with
J1yadjusting base rotation φ,J1xadjusting TCM curvature
κ,J2yadjusting pivot joint orientation αand J2xadjusting
any cable actuated tools attached.
III. MODELING AND ANALYSIS
Tip-based control requires a mapping between the cable
displacement used to actuate arm movement, and the tip
orientation and position in space. If constant-curvature kine-
matics is assumed for the arc of the TCM joint, then the arc
can be considered as being made up of finite curved links
with arc parameters, which can be represented as analytical
frame transformations. This allows for the kinematics to be
decomposed into two mappings, findependent and fspecific.
The first, fspecific, transforms actuator space, q, representing
the cable lengths, to configuration space parameters that de-
scribe the various arm parameters. And the other, findependent,
transforms the configuration space to the task space, describing
the position and orientation of the tip of the device.
A. Robot-Independent Mapping
This section defines findependent as a homogeneous transfor-
mation matrix, T, parametrized by (κ, φ, α, l1, l2, l3), giving
(a)
(b)
Fig. 7. (a) When φ= 0, points A,B,Cand Dlie on the x−zplane. (b)
The angle φrotates the arm out of the x−zplane, as well as rotating the
pivot joint rotation plane around the vector made between points Band C.
task space coordinates x=T1...3,4. The forward kinematics
of the arm can be determined via its geometry, illustrated in
Fig. 7a. From this, we consider the +z-axis to be tangent to
the base of the arm. When φ= 0, adjusting κwill produce
bending movement around the +y-axis, with the tip moving
on the +x-z plane when κ > 0.
From [16], the transformation matrix of a TCM segment
parameterized by (κ, φ, l1)is as shown in (1).
TAB =
cos2φ σ1+1 sin φcos φ σ1cos φsin κl1
−σ1cos φ
κ
sin φcos φ σ1−σ1cos2φ+cos κl1sin φsin κl1
−σ1sin φ
κ
−cos φsin κl1−sin φsin κl1cos κl1
sin κl1
κ
0 0 0 1
(1)
Where: σ1= cos κl1−1
This model takes into account, the θ=κl1rotation about
the +y-axis, the φrotation about the base and the −φrotation
about the tip due to the actuation method, as shown in Fig. 7b.
To transform to point Cas shown in Fig. 7, a translation by
l2in the +z-axis is made. And to get the overall transform,
T, the transformation to point Cis rotated about the +y-axis
by αand translated by l3in the +zdirection, as shown in (2).
α(κ, φ, l1, lD, lE) =
cos−1l02
D+l02
E
8.4if l0
D≤5and l0
E≤5
cos−1−0.118 ((l0
D−2)2+ 0.412 l02
E−24.296)if l0
D>5and l0
E≤5
cos−10.049 (−l02
D−2.428(l0
E−2)2+ 59.006)if l0
D≤5and l0
E>5
(9)
T=TAB hI[0 0 l2]>
0 1 i
| {z }
Translation by l2
Ry(α) 0
0 1
| {z }
Rotation by α
hI[0 0 l3]>
0 1 i
| {z }
Translation by l3
(2)
Once calculated, this gives (3).
T=
cos(α)σ6−σ7sin(2φ)σ8
2
cos(α)σ8σ10 −σ9sin(α) cos(κl1)−cos(φ)2σ8
−cos(κl1) sin(α)−σ5−σ9
0 0
σ2l3σ2+l2sin(κl1) cos(φ)−cos(φ)σ8
κ
σ3l3σ3+l2σ9−sin(φ)σ8
κ
σ4l2cos(κl1) + l3σ4+sin(κl1)
κ
0 1
(3)
Where: σ2= sin(α)σ6+σ5
σ3= sin(κl1) cos(α) sin(φ) + sin(α)σ7σ10
σ4= cos(κl1) cos(α)−σ7
σ5= sin(κl1) cos(α) cos(φ)
σ6=σ7cos(φ)2+ 1
σ7= sin(κl1) cos(φ) sin(α)
σ8= cos(κl1)−1
σ9= sin(κl1) sin(φ)
σ10 = cos(φ) sin(φ)
B. Robot-Specific Mapping
The set of equations making up robot-specific map-
ping fspecific, map from actuator space qto configura-
tion space (κ, φ, α, l1, l2, l3). The actuator space parameters
are the wire lengths, lA,lB,lC,lDand lE, and the dis-
tance from the center to the cable guide, d, as shown in
Fig. 8. From [16], expressions were derived for parameters
(φ(lA, lB, lC), κ(lA, lB, lC), l1(lA, lB, lC)), as shown in (4),
(5) and (6).
φ(lA, lB, lC) = tan−1√3(lB+lC−2lA)
3(lB−lC)(4)
κ(lA, lB, lC) = 2pl2
A+l2
B+l2
C−lAlB−lAlC−lBlC
d(lA+lB+lC)(5)
l1(lA, lB, lC) = 3d(lA+lB+lC)
2pl2
A+l2
B+l2
C−lAlB−lAlC−lBlC
sin−1pl2
A+l2
B+l2
C−lAlB−lAlC−lBlC
6d(6)
𝑑
𝐴
𝐶
𝐵
𝐷𝐸
𝜙
Fig. 8. A distributed wire spacing disc used in the TCM segment, separating
wires A, B and C by 120 deg and wires D and E by 180 deg, with E at 0 deg
and A at 90 deg when φ= 0. The distance of each guide from the center is
represented by d, and the disc rotation is represented by φ.
Configuration space parameters, l2and l3, are constants
associated with the design of the arm and cannot be determined
by actuator space parameters. Pivot joint rotation, α, can be
calculated from parameters (κ, φ, d, l1, lD, lE)as a piecewise
function due to the geometry of the pivot joint itself. If α
exceeds ±22 deg, due to the restriction of the wire guide, one
of the cables will be forced to bend around the edge of the
pivoting section. To derive an expression for α, wire lengths,
lDand lE, were broken up into TCM segment lengths, l00
Dand
l00
E, and pivot joint lengths, l0
Dand l0
E. Representing the overall
lengths as: lD=l00
D+l2+l0
Dand lE=l00
E+l2+l0
E, and
in combination with models derived in [16], the pivot lengths
are calculated using (7) and (8).
l0
D=lD−l2−6 sin l1κ
61
κ+dcos(φ)(7)
l0
E=lE−l2−6 sin l1κ
61
κ−dcos(φ)(8)
Therefore, assuming a maximum αof ±90 deg, αcan be
derived from geometry shown in Fig. 3 and Fig. 8 to get (9).
C. Robot-Independent Rotation About the Backbone
To add an additional DOF to the pivot joint, a manual
rotation around the base can be added to the transformation
matrix shown in the Robot-Independent Mapping section. This
is done by rotating the whole system, calculated in (2), by β
around the +z-axis at the base, as shown in (10).
TM=Rz(β) 0
0 1
| {z }
Rotation by β
T(10)
The product of the first two matrices in (10) is explicitly
shown in (11). Comparing the last two columns of (11) with
(1), it can be shown that both ends of the TCM segment
(a) (b)
Fig. 9. To track the trajectory to the tip (marked by a red dot), the arm was
filmed from the x−y,x−zand y−zplanes, and the tip position was
measured from the axis (in green) for each frame. This figure shows the first
and last frames in the y−zplane when κis varied from 0 to 0.157.
arc can be kept fixed by adjusting φfor changes in β. This
demonstrates the ability to keep the tool steady when rotating
the device manually.
Rz(β) 0
0 1
|{z }
Rotation by β
TAB =
cos(β)σ14 −σ12 σ13 −sin(β)σ11
sin(β)σ14 +σ13 cos(β)σ11 +σ12
−sin(κl1) cos(φ)−sin(κl1) sin(φ)
0 0
cos(β+φ) sin(κl1)−cos(β+φ)σ5
κ
sin(β+φ) sin(κl1)−sin(β+φ)σ15
κ
cos(κl1)sin(κl1)
κ
0 1
(11)
Where: σ11 = cos(κl1)−cos(φ)2σ15
σ12 = cos(φ) sin(β) sin(φ)σ15
σ13 = cos(β) cos(φ) sin(φ)σ15
σ14 =σ15 cos(φ)2+ 1
σ15 = cos(κl1)−1
IV. PRELIMINARY EVA LUATIO NS
To validate the kinematic model produced, the expected
position of the tip was compared to experimental data collected
via cameras setup facing the x-y,x-z, and y-z planes. The
reachability of the arm was then validated using this kinematic
model, by modeling the accessible workspace of the end-
effector for seven separate entry vectors against a simplified
model of the joint capsule.
A. Evaluation of Position and Orientation-Ability
Using cameras facing the x-y,x-z, and y-z planes, the tip
position of the arm was recorded as the curvature was varied
with φ= 0 and α= 0. To obtain position data, each frame
was analyzed using a scale in the background to measure tip
location, as shown in Fig. 9. The results in Fig. 10, show the
relationship between theoretical and experimental data. From
the y-z and x-y planes, a maximum deviation of approximately
5 mm perpendicular to the bending plane can be seen, which
reduces the tip length in the z-direction by approximately 2
mm compared to theoretical results.
This deviation was determined to be due to prototype
limitations. The spacer discs restraining the cables in the TCM
segment have 5 separate holes to run the cable through on each
disc. In our prototype, we allowed too much space to run the
cables through, resulting in a twisting rotational movement
around the device when cables were under tension. In future
versions, smaller holes will be used in cable separation.
Rotation of the pivot joint was also evaluated and showed
smooth movement with little twisting movement.
B. Workspace Validation
To demonstrate the reachable workspace of the tool with
manual rotation around the base, an attached surgical end-
effector length of 4mm was assumed. Accessibility of the
tool was then demonstrated by overlaying the field of ac-
cess of the arm at a fixed entry point along seven separate
insertion vectors in a simplified model of the joint capsule.
Fig. 11 demonstrates that the majority of the workspace can
be accessed with only seven reorientations of the tool, by
using five cross sections along the x-y axis with blue markers
representing accessible space.
V. CONCLUSION
This paper proposes the novel application of cable-actuated
arms in arthroscopy of the hip. Using a TCM segment and
a pivot joint in series, a hand-held prototype was developed
for use with low-force surgical end-effectors or small rolling
shutter cameras. The design and control scheme is described.
The benefit of adding an additional DOF to the tip is evaluated.
It has been demonstrated that the kinematic modeling devel-
oped is appropriate, as well as the suitability for camera end-
effectors due to the arm’s reachability and dexterity to areas
of the surgical workspace, however, further improvements can
be made to enhance the feasibility of the proposed design.
The current control method does not use tip-based control,
and instead directly controls the TCM curvature and base
rotation, as well as tool actuation and pivot joint rotation.
Implementing the kinematics model demonstrated in this re-
port for tip control, as well as including a system for TCM
position stabilization using the on-board IMU would greatly
improve the usability and maneuverability of the device. The
positioning accuracy could be greatly improved by replacing
the current servo motor system with linear actuators. With an
improved prototype, the future work will further validate the
developed ideas in cadaveric experiments.
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---theoretical trajectory --- experimental trajectory ⃝origin
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