Product Design Specification - (Ver. 10)
April 21, 2007
Team Members: Mitch Barnett, Adrienne Belasco, Urvi Patel, Daniel Wilkinson
Advisors: Richard Debski, PhD, Jan Grudziak, PhD, MD, Prerana Patel, MD
Product Design Specification
Bone Screw System for Slipped Capital Femoral Epiphysis Applications
Purpose
The purpose of this document is to provide criteria for the design of a novel titanium bone screw
system that will be used for the treatment of slipped capital femoral epiphysis (SCFE).
Function
Softening of the epiphyseal cartilage during adolescence occurs in SCFE, and the cap of the
femoral head slips off the epiphyseal growth plate at the neck of the femur. Severe grades of
slipping have the risk of avascular necrosis or chondrolysis (Zacher and Gursche, 2003). The
treatment for SCFE is surgery during which the hip slip is repositioned and stabilized by
inserting a metal screw through the growth plate in the cap of the femoral head, closing the
growth plate. The major goal of the treatment is to prevent further slipping (Morrissy, 1990). A
titanium alloy bone screw will be designed to re-stabilize the coxal joint which can be safely
removed in the case of overgrowth or invasion of the joint capsule. A stainless steel
complementary device will be made to remove the screw.
Drawbacks with Current Screws
The current procedure for SCFE treatment has several drawbacks. The drawbacks range from
increased surgical time to increased patient recovery. The current treatment involves the use of
cannulated titanium and stainless steel screws. Removing the screw has several undesirable
aspects. For instance, the hexagonal socket of the screw head often does not allow for an
adequate torque application to remove the screw from the bone (Ilchmann and Parsch, 2006).
Furthermore, some patients sustain fractures after screw removal due to increased stress on the
bone during surgery (Ward, Oct 2007).
Stripping of the screw head during removal occurs more often with titanium than with
stainless steel. A primary cause for this is that the elastic modulus of titanium is closer to bone
than to stainless steel, and thus the bone adheres more strongly to titanium (Patel P, Nov 2007).
The bone grows onto the titanium screw, providing increased support during the healing process
over stainless steel. However, stainless steel is used more frequently due to ease of removal
(Ward, Oct 2007). A drawback with stainless steel is that it produces artifacts in MRI.
Therefore, it is difficult to diagnose a penetrated joint capsule or avascular necrosis (Ward, Oct
2007).
Client Requirements
 Must be suitable for both stable (can walk) and unstable (cannot walk) SCFE
 Must provide an efficient method of screw excision from the implantation site
 Must provide equivalent healing capacity to the current screws on the market
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Product Design Specification - (Ver. 10)
April 21, 2007
Team Members: Mitch Barnett, Adrienne Belasco, Urvi Patel, Daniel Wilkinson
Advisors: Richard Debski, PhD, Jan Grudziak, PhD, MD, Prerana Patel, MD
Design Requirements
1. Physical and Operational Characteristics
a. Performance Requirements: The implantable screw must meet the client requirements as
listed above.
b. Safety: The screw must be biocompatible with the microenvironment of the bone and
must adhere properly to the femur and femoral head to uphold proper healing of the
fracture. The surgeon must have training for usage. The screw system must comply with
FDA guidelines (21 CFR, Subchapter H, Part 888).
c. Accuracy and Reliability: It is the surgeon’s responsibility to use proper surgical technique
for screw insertion and removal. The removal device will fit precisely with the screw head
and will give the surgeon the proper leverage for removal of the screw. He should not have
to exert undesirable torques (more than 30Nm).
d. Postproduction: The screw and removal device may be used once it has been unpackaged
and steam autoclaved at 240ºF (ISO 13402:1995). The screw must be disposed after
removal from the patient.
e. Environment
Storage Environment
Temperature: Store at room temperature.
Operating Environment
Interface: Titanium alloy (Ti6AlNb7) has an elastic modulus similar to that of
regenerating bone, which will maximize adhesion.
Temperature: Temperature of implantation site must be similar to that of native
bone to prevent bone necrosis.
Stress Tolerance/Threshold: Stress in the bone must be minimized during
regeneration so as not to induce further fracture.
f. Screw Size: The shaft diameter will be 5.2mm (±0.02mm) and the length of shaft 55 mm
(±0.5mm). The head of the screw will be 5mm long with a 4.7mm (±0.002mm) hexagonal
depression descending 3mm (±0.01016mm) into the head. A 3mm (±0.01016mm)
hexagonal depression will descend the remaining length of the shaft (9mm ±0.1mm).
Threads will extend 20mm (±0.5mm) up the shaft at a pitch of 2mm.
g. Removal Device: The device will have a T-shaped handle that is 60mm (±0.5mm) across
and 100mm (±0.5mm) long. The hexagonal extension will fit the 9mm depression of the
screw.
h. Weight: The screw will be 3.94 grams (± 0.05 grams), and the removal device will be
33.26 grams (± 0.5 grams).
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Product Design Specification - (Ver. 10)
April 21, 2007
Team Members: Mitch Barnett, Adrienne Belasco, Urvi Patel, Daniel Wilkinson
Advisors: Richard Debski, PhD, Jan Grudziak, PhD, MD, Prerana Patel, MD
i. Materials: Titanium alloy (Ti6Al7Nb) will be used for the bone screw and stainless steel
17-4 PH will be used for the removal device. The testing of the screw will require a steel
fixation device and adapters to fit the Instron 8874. Prototype screws for preliminary
clinical advice will be made with stereolithography.
j. Packaging: Bone screws should be in a closed package. If the package is damaged in any
way it should not be used. Furthermore, all screws and corresponding instruments must be
steam autoclaved prior to surgical use (see postproduction).
k. Disposal: Surgical implants should not be reused and should be properly disposed of in a
biohazard solid waste bin. Also, the removal device should be disposed of in a biohazard
solid waste bin after experiencing visible deformation.
l. Cleaning: Steam autoclave with other surgical equipment at approximately 240ºF (ISO
13402:1995).
2. Production Characteristics
a. Target Design and Development Cost: Less than $1,500 over the course of the eightmonth research and development process.
b. Competition: Synthes, Smith & Nephew, Inc., Stryker Orthopaedics, and AdvaMed are a
few companies that make a variety of cannulated bone screws and extractor devices.
c. Machining Process: The screw and removal device will be manufactured at the Swanson
Center, and the detailing will be completed at Tipton Mold and Die Company with wire
electrical discharge machining (EDM).
3. Testing Characteristics
a. Computational Testing: Finite element analysis will be performed with COSMOSWorks
2007 to choose an appropriate novel design and compare it with the predicate design.
b. Physical Testing: Physical testing will be performed on test parts due to the economic
constraints of machining full screws. The testing will be performed at the University of
Pittsburgh Knee Biomechanics Laboratory under the advisement of Dr. Muturi Muriuki on
the Instron 8874 robot. The robot will apply a torque until a failure mode is experienced,
the torque at which this occurs will be recorded for analysis.
c. Screw Test Part Characteristics: The test part will be a 1 inch cylinder with filleted (1mm)
edges. A 5mm hexagonal socket will descend 3mm into the shaft from both ends and a
3mm hexagonal socket will pass through the entire length of the part. Flat edges will be
drilled 90 degrees apart of the surface of the shaft to allow for immobilization with set
screws.
3
Product Design Specification - (Ver. 10)
April 21, 2007
Team Members: Mitch Barnett, Adrienne Belasco, Urvi Patel, Daniel Wilkinson
Advisors: Richard Debski, PhD, Jan Grudziak, PhD, MD, Prerana Patel, MD
d. Removal Test Part Characteristics: The removal part will be made out of stainless steel
17-4 PH. It will be dual-ended with the predicate removal design on one side (5mm
hexagonal extrude of 3mm), and the novel design on the other (the predicate design with a
3mm hexagonal extrude of 9mm extending off of the base). The removal designs will be
connected by a 10mm solid steel cylinder with a flat edge machined half way between the
removal pieces to allow for immobilization with a set screw.
4. Miscellaneous
a. Standards and Specifications:
ASTM F543-02e1, Standard Specification and Test Methods for Metallic Medical Bone
Screws.
ISO 13402:1995, Surgical and dental hand instruments—Determination of resistance
against autoclaving, corrosion and thermal exposure.
ASTM F382-99(2003)e1, Standard Specification and Test Method for Metallic Bone
Plates.
Also, the bone screw system will require approval by the FDA according to 21 CFR
820.30. The specific regulatory design control documents for the bone screw system and
surgical applications fall under 21 CFR, subchapter H (medical devices), part 888
(orthopaedic devices). More specific regulations under orthopaedic devices include 21
CFR 888.4540, 3040, and 3030. Approval will be determined on the basis of initial
hazard analyses, verification and validation testing, a human factors evaluation, and
material and mechanical testing.
b. Customer: Surgeons and hospital purchasing managers.
i. Instructions: The screw and its removal device will come with detailed instructions in
terms of usage and an application manual.
ii. Constraints: Titanium screws must be compatible with commercially available bone
plates (ASTM F382-99(2003)e1).
iii. Patient-related Concerns: The patient must not have product-related allergies or
take medicine that would complicate their recovery period.
References
Ilchmann, T., Parsch, K. “Complications at screw removal in slipped capital femoral epiphysis
treated by cannulated titanium screws.” Archives of Orthopedic and Trauma Surgery
(2006), 126: pp. 359-363.
Morrisy, RT. Slipped Capital Femoral Epiphysis (Chapter 24). Lovell and Winter’s Pediatric
Orthopedics, third edition. Philadelphia, JB Lippincott Co. (1990), pp. 885-902.
Ward, Timothy. Clinical Interview. 24 Oct. 2007.
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Product Design Specification - (Ver. 10)
April 21, 2007
Team Members: Mitch Barnett, Adrienne Belasco, Urvi Patel, Daniel Wilkinson
Advisors: Richard Debski, PhD, Jan Grudziak, PhD, MD, Prerana Patel, MD
Patel, Prerna. Clinical Interview. 25 Nov. 2007.
Zacher, J and Gursche, A. “Hip Pain.” Best Practice and Research Clinical Rheumatology.
(2003), 17:1, pp. 71-85.
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