Total Knee Prothesis Size: some hints

source (copy-paste): this website

tkr1When total knee replacement was in its infancy in the early 1970s, the manufacturers of the implants offered the prostheses in very limited sizes. At surgery the patient’s bone had to be sculptured so as to fit the available sizes of implants. It rapidly became obvious, however, that a better scenario would be to have a variety of implant sizes that would come close to matching the patient’s anatomical situation. Also, it was obvious that sizing was more than just designing implants for tall or short patients, male or female patients, or even for thin or heavy patients.

At present, all of the prosthetic implant companies offer knee prostheses in a variety of sizes. How were these sizes determined? Initially, designers studied the anatomy of normal femoral and tibial bones in the pathology laboratory. Subsequently, with the advent of CT scans, these were used to compile a database of the “standard” sizes and shapes of the bone of patients who were going to undergo knee replacement. There were also evaluations of the width and height of the joint surfaces that were used to determine implant sizes. This method of designing implants is a good one as long as the “database” of CT scans or bones that are evaluated is representative of the patients that will be operated upon. There are subtle variations between various ethnic groups throughout the world. Therefore, the more diverse patient bones that are studied, the more valid is the database.

tkr2The result has been that all modern, reputable manufacturers of prostheses offer implants that cover over 98% of the sizes that we see in clinical practice. For those in the remaining 2%, a custom implant may be required.  This is very much akin to sizing for shoes. If your size is somewhere between 5A and 14D, you can probably buy a pair of shoes over the counter. If you lie beyond these “norms” (not meant in any pejorative sense), then you may need a custom product. Is there a downside to having a custom implant for each and every patient? Cost is obviously a consideration, but more than that, there is the potential error involved in fabricating any custom product.

how to determine the proper implant size?

Traditionally, the surgeon has used plastic overlay templates that are placed over the pre-operative x-rays. The templates are corrected for the magnification that can occur when the x-ray is taken. By comparing a variety of template sizes and shapes, the surgeon can get a strong suggestion of the implant that is needed. The final determination of size is made at surgery. The size and shape of the bones are measured during the surgical procedure and from these measurements the surgeon can judge how much bone to remove. When the manufacturers of the implants provide surgeons with a series of trial prostheses, these can be placed on the bone so as to assess for proper fit and soft tissue balance. At that point small changes to the bony cuts can be made, if necessary, and soft tissue releases performed to assure that the implants fit well and that the soft tissue (the capsular sleeve) is well balanced. Finally, permanent implants the same size as the appropriate trial implants are chosen, and these are then affixed to the bone ends.

tkr3Pre-operative templating is possible electronically. Using the digital images from radiology with digitized templates, the surgeon can assess implant sizes and perform a computer simulation of the insertion of the prosthesis. The surgeon can assess effects of changing implant sizes, changing implant designs (posterior stabilized vs. posterior cruciate ligament retaining prostheses), and even implants from different manufacturers. Thus, it is possible to identify in advance which patient will require a specially-sized – or even a custom – implant, and these can then be fabricated by the manufacturer.

gonna feel happy in 3, 2, 1 …

… GO! In this precise moment, you’re feeling an inexplicable feeling of happiness, aren’t you? Oh yes, I knew that. How? Simple, I computed it, and maths is not an opinion.

The equation reported a few lines below here is the expression of a computational model of momentary subjective well-being proposed in this article. Basically, it allows to compute momentary happiness based on expectations, rewards and past outcomes. The study was conducted on 26 subjects who had to deal with monetary options. In each trial, each subject had to choose between a certain option (win the sum X) and a gamble (gamble the sum Y). “Chosen gambles were resolved after a 6-s-delay period. Every two to three trials, subjects were asked to indicate” how happy they were at that precise moment.


I’ll leave you the pleasure to read the article for more details; in this post, we’ll simply take a look at the equation terms and try to understand them 🙂

  • the variable t is not the time, but the current trial (decision whether to win or gamble). The variable j is a past trial;
  • the terms w0, w1, w2 and w3 are weights assigned to the influence estimated for different event types;
  • the forgetting factor 0 ≤ γ ≤ 1 makes “events in more recent trials more influential than those in earlier trials“;
  • CR means Certain Reward, the certain option, while EV is the Expected Value of the alternative gamble. So, CRj and EVj are the average rewards of the trial j;
  • RPEj is the difference between experienced and predicted rewards “on trial j contingent on choice of the gamble“.

Basically, at each trial j, if CRj is chosen then EVj = 0 and RPEj = 0; but if the subject chooses the gamble, then only CRj = 0, while EVj and RPEj are based on previous experience. In any case, these three quantities (CR, EV and RPE) are all linked to dopamine activity. Dopamine is a neurotransmitter (chemical released by nerve cells) which, in human brain, is responsible for sending signals to other nerve cells. The researchers “hypothesized that these dopamine-related quantities might explain momentary happiness“.

left-prefrontal-cortexUsing computational modeling, it was shown that “emotional reactivity in the form of momentary happiness in response to outcomes of a probabilistic reward task is explained not by current task earnings, but by the combined influence of recent reward expectations and prediction errors arising from those expectations“. In more general terms, happiness can be modeled as emotional reactivity to recent rewards and expectations. This allows to investigate the neural mechanisms which underlie the relationship between ongoing happiness and life events.

The researchers claim that their key finding is that “happiness is related to quantities associated with temporal difference errors that phasic dopamine release is thought to represent. At the very least, this hints at a link between dopamine and emotional state, consistent with suggestions that this neuromodulator plays a role in mood regulation in healthy and depressed subjects“. Upon further investigation, any scientific evidence that emotional state is related to brain and behavior could provide a framework for the development of model-based assays for the interpretation of mood disorders.

PPRIME Forum “Mechanical Design and Mechatronics of Robotics Systems”

@ Futuroscope (Poitiers), November 6 and 19, 2014

Lecture hall, SP2MI building – avenue Gustave Eiffel, Chasseneuil, France


The FORUM organized by the Pprime Institute offers the opportunity to PhD students and researchers from the robotics community to meet experts in order to exchange with them about most recent scientific results. This event provides to participants a space of reflection and privileged exchange. The FORUM is dedicated to Mechanical Design and Mechatronics of robotics systems.

Two themes, in the field of interest of the robotics team of Pprime institute, are considered during this FORUM:

  • the design of mechanical hands for dexterous manipulation;
  • the design of complex poly-articulated mechanisms (parallel mechanisms).

The forum is organized over three days with a program focused on presentations and panel discussions. In this context, the FORUM will have four guest speakers:

  • Yukio Takeda, Professor, Tokyo Institute of Technology, Japan
  • Philippe Wenger, Directeur de recherches au CNRS, IRCCyN-Nantes, France
  • Markus Grebenstein, doctor, DLR German Aerospace Center, Munich, Germany
  • Chin-Hsing Kuo, Professor, National Taiwan University of Science and Technology, Taiwan

Program Overview

November 6: «Kinematic optimization of complex poly-articulated systems »

  • 10h-12h: Yukio Takeda, “Kinematic Design of Compensatable Parallel Manipulators
  • 14h-17h: Philippe Wenger, “Coping with singularities in the design of parallel manipulators
    Yukio Takeda, “Kinematic and Dynamic Analysis and Design of 3-RPSR Parallel Mechanism for Pipe-Bender

November 19: «Design of medical robots »

  • 10h-12h: Chin-Hsing Kuo, “Applications of Mechanism Design Theories for Surgical Robotics
  • 14h-17h: Discussion Panel “How to choose the suitable kinematic design for a given application? What is the best approach for kinematic optimization?

Registration is free but required for logistic reasons on the FORUM website.