Heated Bolt for Modular Hip Stem Attachment
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Solution Overview
Problem
In hip replacement surgery, achieving a strong and durable attachment between the proximal and distal stems of a modular hip prosthesis without over-torquing, which can cause damage, is challenging due to the limitations of traditional tightening methods.
Innovation Solution
The method involves heating the bolt to an elevated temperature above human body temperature, tightening it to a specified torque, and then allowing it to cool in place, which induces tensile stress and increases the compressive force between the stems, enhancing the attachment strength beyond what can be achieved by torque alone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the bolt is tightened to a specified torque during surgery, then the attachment strength is sufficient for basic fixation, but the attachment strength cannot be increased further without over-torquing and causing damage
Solution Approach 1:
The patent applies parameter changes by heating the bolt to an elevated temperature before tightening, which changes the physical state of the bolt material. This temperature parameter change allows the bolt to be tightened at a lower effective stress level, and subsequent cooling creates thermal contraction that increases the compressive force and attachment strength beyond what could be achieved by torque alone without causing over-torquing damage.
Solution Approach 2:
The patent directly utilizes thermal expansion principles by heating the bolt before installation. The bolt is heated to expand thermally, then tightened while hot, and finally allowed to cool and contract, creating a tensile stress in the bolt that translates to increased compressive force at the attachment interface. This thermal expansion and contraction cycle enables enhanced attachment strength without increasing the tightening torque.
2Strength
If the bolt is heated to elevated temperature and tightened, then the attachment strength is significantly increased, but the process complexity increases
Solution Approach 1:
The patent applies preliminary action by heating the bolt to an elevated temperature before the tightening operation. This preparatory heating step is performed in advance of the actual assembly, allowing the bolt to be installed while hot and then cool in place to develop the desired thermal contraction and enhanced attachment strength. The preliminary heating action separates the thermal preparation from the mechanical assembly steps.
Solution Approach 2:
The patent utilizes phase transitions in the form of thermal phase changes - heating the bolt from ambient temperature to an elevated temperature state, maintaining it in that state during tightening, then allowing it to cool and contract. This controlled thermal phase transition enables the enhancement of attachment properties without requiring complex mechanical modification of the bolt or assembly process.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly increases the holding strength of the bolt attachment between the proximal and distal stems, providing a more secure fixation of the modular hip stem within the patient's body.
Implementation Method 1
If the bolt were unattached during cooling, it would longitudinally shrink due to the effects of thermal expansion. Because the bolt is attached at an elevated temperature and cools in place, it experiences a tensile stress due to the effects of thermal expansion.
Data Source
AI summary
During hip replacement surgery, a practitioner can attach a distal stem to a femur of the patient and can then fixedly attach a proximal body to the distal stem using a bolt. Prior to tightening, the bolt can be heated to an elevated temperature greater than average human core body temperature. The practitioner can tighten the bolt to a specified torque while the bolt is at the elevated temperature. After tightening, the bolt cools to average human core body temperature and experiences a tensile stress due to the effects of thermal expansion. The tensile stress in the bolt produces a compressive force between the distal stem and the proximal body. The compressive force can increase the attachment strength of the bolt to the distal stem and the proximal body, beyond what can be achieved by solely torquing the bolt to the specified level during surgery without first heating the bolt.