Osteoarthritis: some hints

sources: Orthopaedic Research Society and PR Newswire

According to Wikipedia, Osteoarthritis (OA) is a type of joint disease that results from breakdown of joint cartilage and underlying bone. The most common symptoms are joint pain and stiffness. Initially, symptoms may occur only following exercise, but over time may become constant. Other symptoms may include joint swelling, decreased range of motion, and when the back is affected weakness or numbness of the arms and legs. The most commonly involved joints are those near the ends of the fingers, at the base of the thumb, neck, lower back, knee, and hips. Joints on one side of the body are often more affected than those on the other. Usually the symptoms come on over years. It can affect work and normal daily activities. Unlike other types of arthritis, only the joints are typically affected.

OA affects the entire joint, progressively destroying the articular cartilage, including damage to the bone. Patients suffering from OA have decreased mobility as the disease progresses, eventually requiring a joint replacement since cartilage does not heal or regenerate. According to a 2010 Cleveland Clinic study, OA is the most prevalent form of arthritis in the United States, affecting more than 70% of adults between 55 and 78 years of age (that is, millions of people).

“My father was in major pain from his osteoarthritis,” explains Riccardo Gottardi, a scientist at the University of Pittsburgh supported by a Ri.MED Foundation fellowship.  “He was in so much pain that he had to undergo a double hip replacement followed by a knee replacement soon afterwards. I could see the debilitating and disabling effects the disease had on him, as he was restricted in his mobility and never fully recovered even after surgery. This was very different from the person that I knew, who had always been active and never shied away from long hours of work in his life – he just could not do it anymore.“

For scientists like Gottardi, a key obstacle in understanding the mechanisms of osteoarthritis and finding drugs that could heal cartilage, is that cartilage does not exist separately from the rest of the body. Cartilage interacts with other tissues of the joint, especially with bone. Bone and cartilage strongly influence each other and this needs to be taken into account when developing new drugs and therapies.

Gottardi and a team of researchers at the Center for Cellular and Molecular Engineering, led by Dr. Rocky Tuan, have developed a new generation system to produce engineered cartilage, bone and vasculature, organized in the same manner as they are found in the human joint.  This system is able to produce a high number of identical composite tissues starting from human cells. The team will use this system to study the interactions of cartilage with vascularized bone to identify potential treatments for osteoarthritis. The team’s research has two main objectives: to help understand how cartilage interacts with the other joint tissues, especially bone; and to help develop new effective treatments that could stop or even reverse the disease.  Their patent pending system is the first of its kind, and offers a number of advantages including the use of human cells that replicate native tissues. This system more closely matches the effects on humans than standard animal testing could achieve.

The team of scientists is further developing their system to produce tissues composed of more and different cell types that could better replicate the human joint. They have also started a number of collaborations with other research groups and companies that are interested in using the system to investigate other joint diseases and to test their product. “After seeing what my father went through,” says Gottardi, “I decided that I did not want to just watch by working on diagnostics, but rather, I wanted to be able to do something about osteoarthritis and contribute to the improvement of current treatment options.“

Gottardi’s work was recently presented at the Annual Meeting of the Orthopaedic Research Society. Founded in 1954, the Orthopaedic Research Society strives to be the world’s leading forum for the dissemination of new musculoskeletal research findings.

Formation à la chirurgie robotique

source: ce site internet

Saviez-vous qu’il n’y a aucune obligation légale à se former à la chirurgie robotique ? Autrement dit, n’importe quel chirurgien peut s’assoir à une console de robot da Vinci et opérer. Il n’y a aucune obligation de formation. Prendriez-vous un avion dont le pilote n’a pas de licence de vol ?

Le 13 novembre 2015, un panel d’experts de la chirurgie robotique s’est réuni au sein de l’Académie Nationale de Chirurgie (ANC) pour débattre sur la formation en chirurgie robotique. Les conclusions principales mettent en avant des besoins fondamentaux :

• Il faut se caler sur le protocole de formation à la chirurgie robotique élaboré par les équipes de Nancy.
• Il est impératif de prévoir l’arrivée de nouveaux robots.
• Il n’est pas utile de multiplier les centres de formation, il faut se contenter de quelques centres experts de formation bien équipés en matériel et en personnel et développer les registres.

Les principes conducteurs de la chirurgie moderne assistée par ordinateur, et par conséquent robotique doivent être les suivants :

• Savoir opérer
• Connaître les machines utilisées
• Être convaincu de la nécessité du partenariat avec les industriels
• Conserver son éthique et son indépendance en évitant tout conflit d’intérêts
• Justifier scientifiquement les évolutions thérapeutiques choisies pour convaincre les décideurs économiques et défendre les chirurgiens mis en cause par l’intermédiaire de la HAS

La formation en chirurgie robotique assurée actuellement par les industriels n’a pas de base légale. Celui-ci a simplement pour obligation, comme tout fabricant de matériel, d’expliquer son fonctionnement à un acquéreur. Cette formation, trop courte si l’on se réfère aux résultats publiés, ne comporte aucune évaluation des capacités de ces chirurgiens à utiliser le robot. Il est du rôle des sociétés savantes et des universités de contrôler cet enseignement et l’évaluation des équipes utilisatrices de ces nouvelles technologies, en partenariat avec les industriels. La formation en chirurgie robotique peut être assurée soit par des écoles publiques, soit par des écoles privées, en étant conscient de l’importance du coût du matériel nécessaire. Il apparait que ce coût ne peut être assumé par les seules finances des universités et que des partenariats sont nécessaires avec des entreprises privées, pour les écoles publiques.

La chirurgie robotique est mise en œuvre par des chirurgiens et leurs équipes et leur formation comporte 5 volets :

1. La formation chirurgicale de base relève de toutes les écoles de chirurgie.
2. La formation élémentaire à l’usage d’un “robot” est commune à toutes les spécialités utilisatrices des robots. Elle est validée par un document attestant de la participation du chirurgien à la formation élémentaire, apprentissage de la machine, des gestes avec des mises en application sur simulateur et sur robot en “dry” et “wet lab”. Cette étape de la formation doit être conclue par une évaluation.
3. La chirurgie robotique est caractérisée par la distance physique établie entre le chirurgien et le champ opératoire, ainsi que par la disparition de la communication visuelle avec le reste de l’équipe. Une formation des autres membres de l’équipe chirurgicale (team training) est par conséquent indispensable.
4. La formation clinique spécifique à chaque spécialité se fera dans des centres équipés de “robots” et disposant de “proctors” (“Advanced Courses”).
5. La chirurgie est un apprentissage permanent qui nécessite un maintien de compétences tout au long de sa pratique. La question de la recertification telle qu’elle est imposée aux pilotes d’avion après une période d’inactivité ou lorsqu’ils n’ont pas une pratique régulière n’existe pas actuellement en Médecine. Il est vraisemblable qu’à l’avenir le développement des simulateurs permettra aux chirurgiens soumis à des situations similaires de rafraîchir ou de maintenir leur technicité.

Knee Surgery… there’s an app for that!

Nowadays “there’s an app for that” could be the 11th Commandment … there is a bunch of apps for everything, even for the most stupid daily activities.

A friend of mine told me to have a look at this one: Virtual Knee Surgery. It is a very realistic game that follows all the basic steps of a Total Knee Replacement surgery. The user is invited to interact with the surgeon in order to perform the necessary operations.

After living this experience in a real operating room, personally I’ve found this app a really nice and clear explanation 🙂 If you want to spend ten minutes to learn the basics of TKR, I definitely can say it’s worth it!

CORTEX 3D-printed cast

source: this website

3D-printed casts for fractured bones could replace the usual bulky, itchy and smelly plaster or fibreglass ones in this conceptual project by Victoria University of Wellington graduate Jake Evill. The prototype Cortex cast is lightweight, ventilated, washable and thin enough to fit under a shirt sleeve.

A patient would have the bones x-rayed and the outside of the limb 3D-scanned. Computer software would then determine the optimum bespoke shape, with denser support focussed around the fracture itself. The polyamide pieces would be printed on-site and clip into place with fastenings that can’t be undone until the healing process is complete, when they would be taken off with tools at the hospital as normal. Unlike current casts, the materials could then be recycled.

Jake has just graduated from the Architecture and Design faculty at Victoria University of Wellington, with a Major in Media Design and a Minor in Industrial Design. After working with the orthopaedic department of his university on the project, he is now looking for backing to develop the idea further. “At the moment, 3D printing of the cast takes around three hours whereas a plaster cast is three to nine minutes, but requires 24-72 hours to be fully set,” says the designer. “With the improvement of 3D printing, we could see a big reduction in the time it takes to print in the future.”

After many centuries of splints and cumbersome plaster casts that have been the itchy and smelly bane of millions of children, adults and the aged alike, the world over, we at last bring fracture support into the twenty-first century.

The Cortex exoskeletal cast provides a highly technical and trauma-zone-localised support system that is fully ventilated, super light, shower friendly, hygienic, recyclable and even stylish!

other sources: one and two

how do our eyes move ?

Imagine you’re watching a tennis match. Or better, imagine you’re the chair umpire. You must focus on the yellow ball, decide whether or not it is in and continuously follow its trajectory. At the end of the match, your eyes will “feel tired”. That’s because you made them work a lot while following the small yellow moving target. Which kind of movements do our eyes perform while tracking a moving object?

So, the situation is that the tennis ball moves quickly from one side of the court to the other and you want to continuously look at it. You have two possibilities: either you follow the ball by rigidly turning your whole body (or just your head) without changing the direction of your gaze, or you move just your eyes by performing really quick changes of direction. In the first case, you’d look quite weird and at the end of the match you’ll be completely sweat; in the second case, which is the solution we normally adopt in all similar daily life activities,  you’d properly employ your extraocular muscles in order to optimize the control of your eye movements.

When our eyes quickly jump from a position to another, they are performing the so-called saccades. To have an idea, pick the triangle and the star on the left (in the middle, the extraocular muscles are shown!) and jump with your eyes repeatedly from one to the other, as quick as possible: you’re just performing saccadic eye movements (video at the end of this post). Their maximum angular speed is proportional to their length (the distance they have to cover) and can attain up to 1000 deg/sec. A saccade takes 200 milliseconds to initiate and then lasts from 20 to 200 ms. This makes it the fastest movement produced by the human body, even faster than blinking (300-400 ms). Thanks to specific neuronal mechanisms connected to our eye muscles, time-consuming circuits are bypassed and everything works quite automatically (you don’t actually have to think about how to follow a moving object with your eyes, you just do that). All such amazing neuronal mechanisms are so natural and, in a sense, involuntary that saccades appear even in the opposite case, that is for fixational eye movements.

You, the chair umpire, are not sure whether the ball was in or out. Thus, you take the slow motion of the match and you stop the video on the exact instant when the ball touched the ground. Now you start staring at the still yellow target, which is still. After some seconds of prolonged visual fixation, some small, jerk-like, involuntary eye movements, similar to miniature versions of saccades, will occurr. They are called microsaccades and participate to the maintenance of visibility, even if their precise role in visual perception is is still largely unresolved.

Now imagine you’re watching the dvd of your favourite tennis match and you accidentally push the button that slows the video down . Instead of the real velocity, you’re watching the match at 0.2x speed. The ball has a really slow motion and you try to follow it with your eyes. You can perform smooth movements of your pupils, something that is completely different from the saccadic jumps you were obliged to during the match! This voluntary gaze shift to closely follow a moving object is allowed by smooth pursuit eye movements, which are asymmetric: for example, most humans and primates tend to be better at horizontal than vertical smooth pursuit.

Saccades and pursuits are just two of the main types of eye movements (you can find a complete list here).

sources: i, ii, iii, iv

3D-printing human body parts

In a previous post we saw how new technologies aim at printing human body parts by means of 3D printers. Recently, an amazing result has been achieved by some Doctors at the University of Michigan: they managed to print in 3D a tracheal splint for a 20-month-old patient in order to restore his bronchus functionality. This website offers the description of the surgical operation they performed, step by step.

Abstract – Tracheobronchomalacia in newborns, which manifests with dynamic airway collapse and respiratory insufficiency, is difficult to treat. In an infant with tracheobronchomalacia, we implanted a customized, bioresorbable tracheal splint, created with a computer-aided design based on a computed tomographic image of the patient’s airway and fabricated with the use of laser-based three-dimensional printing, to treat this life-threatening condition.

Description – At birth at 35 weeks’ gestation, the patient did not have respiratory distress and otherwise appeared to be in normal health. At 6 weeks of age, he had chest-wall retractions and difficulty feeding. By 2 months of age, his symptoms progressed and he required endotracheal intubation to sustain ventilation. The workup revealed the following:

• an anomalous origin and malposition of the pulmonary arteries, with crisscross anatomy;
• right pulmonary-artery hypoplasia;
• compression of the left mainstem bronchus between an abnormally leftward-coursing ascending aorta and an anteriorly displaced descending aorta;
• air trapping;
• postobstructive pneumonia.

Despite placement of a tracheostomy tube, mechanical ventilation, and sedation, ventilation that was sufficient to prevent recurring cardiopulmonary arrests could not be maintained.

We reasoned that the localized tracheobronchomalacia was the cause of this physiological abnormality and made a custom-designed and custom-fabricated resorbable airway splint. Our bellowed topology design, similar to the hose of a vacuum cleaner, provides resistance against collapse while simultaneously allowing flexion, extension, and expansion with growth. The splint was manufactured from polycaprolactone with the use of a three-dimensional printer.

The institutional review board of the University of Michigan consulted with the Food and Drug Administration and approved the use of the device under the emergency-use exemption, and written informed consent was provided by the patient’s parents. After transposition of the right pulmonary artery and failed aortopexy, sutures were placed around the circumference of the malacic left bronchus and tied through interstices of the splint, and the bronchus was expanded. Subsequent bronchoscopy revealed normal patency of the bronchus without dynamic collapse and normal ventilatory variation in the size of the left lung. The partial pressure of carbon dioxide in venous blood decreased from 88 to 48 mm Hg. Seven days after placement of the airway splint, weaning from mechanical ventilation was initiated, and 21 days after the procedure, ventilator support was discontinued entirely and the child was discharged home with the tracheostomy in place. One year after surgery, imaging and endoscopy showed a patent left mainstem bronchus . No unforeseen problems related to the splint have arisen. Full resorption of the splint was estimated to occur in 3 years.

This case shows that high-resolution imaging, computer-aided design, and biomaterial three-dimensional printing together can facilitate the creation of implantable devices for conditions that are anatomically specific for a given patient.

other sources: one and two

Repairing and Replacing Body Parts: What’s Next

A friend of mine sent me the link to this webpage. It’s an interesting article that I simply copy and paste here! Enjoy!

🙂

Advances in medical technology have helped us live longer. Now, researchers are exploring ways to repair, refurbish, or replace human organs that have been damaged by chronic disease, traumatic injury, heart attack, stroke, or just plain aging.

“Medicine is saving people who previously we weren’t able to save,” says Dr. Doris A. Taylor, director of regenerative medicine research at the Texas Heart Institute in Houston. Even so, demand for donor organs exceeds the number available. “Each year thousands of people die while waiting for an organ,” Taylor says. That gap in supply and demand is one factor that has led researchers to ever more innovative treatments; at times these treatments can sound like science fiction come to life. Here’s a look at what repaired and replacement parts are available to patients now, which treatments are undergoing clinical trials, and what medical scientists are working to achieve in the future.

Eyesight

Here’s a new take on magnifying glasses: Surgeons can now implant a tiny telescope within the eye, to help restore some of the vision lost to end-stage age-related macular degeneration (AMD), a disease that affects 1.8 million Americans and is the leading cause of legal blindness for adults age 60 years and older. The device—which the Food and Drug Administration approved in 2010 and which is becoming more widely available to medical institutions across the country—is implanted via an hour-long outpatient procedure under local anesthesia. It requires about a month of working with an occupational therapist to get used to, says Dr. Mark Mannis, director of the Eye Center at the University of California Davis Health System. “The reason is that this is not a simple restoration of vision,” he says. “It really requires the patient to see in another way, much in the same way that a patient who loses a lower limb and then gets a prosthesis needs to learn how to walk in a new way.” In this case, the patient learns to use one eye—the one with the implant—for detailed vision and the other for peripheral vision.

Regenerative Medicine

As director of the Wake Forest Institute for Regenerative Medicine in Winston-Salem, North Carolina, Dr. Anthony Atala is researching treatments to repair or restore—or “regenerate”—damaged or failing tissues and organs by using the patient’s own cells and healing abilities. That can mean “boosting” healing by injecting stem cells, or by implanting tissues or organs that have been artificially bio-engineered in the lab starting with stem cells usually harvested from the patient, a strategy that minimizes the risk of the tissues or organs being rejected. “We’re working on about 30 different tissues and organs,” Atala says. Already a number of implants have been tried successfully in humans, including knee cartilage, skin, blood vessels, urethras, windpipes (trachea), and bladders. Clinical trials are underway to treat urinary incontinence by implanting cells to help boost functionality of the urinary valve and thus keep patients dry. Says Atala: “The future is focused on making sure that these technologies can get to as many patients and as many conditions as possible.”

“Printing” Body Parts

At Atala’s lab and other regenerative medicine research centers, 3-D printing is another experimental strategy being used to build bio-artificial body parts and organs. “We’ve printed ear lobes and nose parts and miniature kidneys and skin,” he says. “You are laying [down] the cells one layer at a time, and placing the cells right where you need them,” by customizing the different layers to form the necessary shape, he says. “If you think of your printer and your ink cartridge, instead of using ink you’re using cells and a gel.” Wake Forest is also investigating the possibility of “printing” skin cells directly onto burn wounds.

Stem Cells for Stroke Recovery

Neurologist Dr. Lawrence Wechsler of the University of Pittsburgh’s Schools of the Health Sciences is in the early stages of exploring whether stem cells, injected directly into the brain, can aid stroke victims in their recovery. The first step—now being tested in a clinical trial—is establishing that it’s safe just to try the technique. If that goes well, Wechsler says, “then we can design a study that will more reasonably look at the issue of efficacy and clinical benefit.” Such therapies wouldn’t “unparalyze” patients, he warns. But small improvements in function could yield big improvements in quality of life. “If you can begin to use your hand to grip something and do some small tasks,” Wechsler says, “or gain enough strength in your leg to help you move from being in a wheelchair to walking in some way, that change is a huge benefit.”

Growing an Artificial Heart

Perhaps the ultimate goal of regenerative medicine researchers is creating and transplanting a functioning bio-artificial heart. Is it feasible? Building complex solid organs like the heart, liver, lungs, and pancreas is challenging, and a major issue will be “where do you get those hundreds of billions of cells to do this,” says Taylor of the Texas Heart Institute. But she adds, “we’re making huge strides,” and predicts that a transplant of one kind of bio-artificial solid complex organ will be possible within five years. “And if I have anything to say about it,” Taylor says, “I will be there when it happens.”

For more, see “On Beyond 100“

we risk freezing down

Seriously, it’s too hot outside. All of a sudden I’ve turned the heating off in my small apartment, started watering plants daily and begun the countdown for the first bath of 2013!

Inspired by such a summerish weather, I thought it could be interesting to read something about Hypothermia and its causes. Hope this post will help cooling down a bit – but not too much, it wouldn’t be nice to freeze down completely 🙂

The first notion to know is that of Body Temperature (BT), the temperature that a living being autonomously keeps more or less constant through biologic processes such as homeostasis or thermoregulation. BT is about 37 °C for a human being, 38.5 °C for a pig, 39 °C for cats and cows, between 34 °C and 40 °C for camels and dromedaries, 42 °C for birds (whoa!) and so on… There is a sort of threshold, different for each single species, for the minimum temperature required to allow normal metabolism and body functions. Now, cows hypothermia is for sure an awesome topic and could raise kind of interesting discussion, but we’ll focus on the human being. For a normal healthy man, the threshold value is 35.0 °C. If our core temperature (i.e. “the temperature of an organism at which it is meant to operate”) drops below such 35 °C, we start feeling bad since our body does not like working in suboptimal conditions.

Wiki says that “Hypothermia usually occurs from exposure to low temperatures, and is frequently complicated by alcohol. Any condition that decreases heat production, increases heat loss, or impairs thermoregulation, however, may contribute”.

That’s the point! We lose our body heat in many ways, as shown in this nice image. Or better, we continouosly exchange our body heat with every single thing that surrounds us in daily life. In a sense, we are nothing more that walking heaters. Our internal mechanisms, normally, are enough to keep a constant BT. But in the case of, for example, prolonged exposure to cold, our body might become unable to replenish the heat that is being lost. As a consequence, a drop in core temperature occurs. This change causes a host of characteristic symptoms (according to the hypothermia degree), such as:

• shivering,
• mental confusion (difficulty in speaking, sluggish thinking, and amnesia),
• muscle mis-coordination,
• cyanosis (exposed extremities become blue),
• decreased heart rate, respiratory rate and blood pressure.

If you ever plan to swim or dive in cold water, to explore snowy landscapes, to drink alcohol and smoke outside at night (alcohol and tobacco -> vasodilatation -> sensation of warmth while, instead, heating loss is rapidly increasing), to chase russians in their homeland in 1812 and so on… well, you’d better take your time and think about all the risks you’re going to take.

A subject found in hypothermic conditions needs to be rewarmed. Rewarming can be achieved in three main ways:

1. passive external rewarming: the subject is moved to a warm environment and provided with properly insulated dry clothing. Then, their own heat generating ability will be enough to restore proper BT conditions.
   
2. active external rewarming: external warming devices, such as warmed forced air or hot water bottles placed in both armpits and groin, are employed to help the subject warming up faster.
3. active internal rewarming: it involves the use of intravenous warmed fluids, irrigation of body cavities with warmed fluids or inhalation of warm humidified air.

In case of severe hypothermia, extracorporeal rewarming such as via a heart lung machine may reveal to be the fastest (and only) solution.

Watch out also for really hot environments! Hyperthermia, the opposite of hypothermia, can lead to heat exhaustion and heat stroke.

sources: one, two and google images

let’s take stock of … the lower limb !

Hello everybody! Since the number of daily readers (and followers) of my blog is (surprisingly) increasing day after day (Thank you everybody!), I thought it could be useful to take stock of some important posts I wrote about the lower limb. Let’s start from the top -the hip- and go down to the bottom -the ankle-, with 9 posts that got many views and some funny comments 🙂

Obviously, since my PhD project is about a knee prosthesis, most of the posts (5 out of 9) are about the knee joint. But in general I tried to give an overall view of some interesting topics related to the biomechanics of the lower limb. Enjoy! 🙂

the Hip Joint: some hints

hammers, screws and Intramedullary nails

the Knee Bursae: some hints

the Meniscus: some hints

the Patella: some hints

Knee Alignment Conditions

Patellar Reflex

How many limbs do you actually perceive?

the Ankle Joint: some hints

How many limbs do you actually perceive?

A famous movie showed us the advantages of having more limbs than usual. A common person with, for example, six independent arms could be stronger than normal, more dexterous and more efficient in killing annoying spiders. The four additional arms would actually be part of his body, normally connected to his motor nerves in order to be very accurately controlled by his brain. He would also feel pain if one of his additional arms was hurt.

Try to figure out the sensation, I mean the physical sensation, of being able to control each single limb of your body. Focus on all the feelings that you can actually experience with your legs and arms (temperature, pain, sense of touch, shape recognition, EVERYTHING!): it’s amazing. Each part of our body is innervated in a way that determines to which extent we “feel” neuro-biological phenomena. Our nerves get grouped in the spinal cord and the data they collect are transmitted up to the brain. In different areas of the primary motor cortex we can actually find a precise mapping of all the different body regions that are directly responsible for the exchange of sensory and motor information. So, for instance, it is possible to localise the motor cortex area corresponding to the tip of our left thumb.

Now, the fact is that innervation of body regions is not uniform at all. Our lips are clearly more sensitive than our elbows. These differences are respected at the level of the motor cortex. Basing on the amount of cerebral tissue or cortex devoted to a given body region, some smart people proposed a funny representation of primary motor cortex anatomical divisions (picture on the left). In this bizarre cortical homunculus, the size of each body region is proportional to how richly innervated that region is, and not to its actual size. This brain representation develops over time and is different for each individual. What is sure is that our brain “feels” the presence of each single part of our body according to this representation at the level of the primary motor cortex. This leads us to the concept of “body within the brain”:  one’s hand, or toe, or nose exist as soon as it is represented by a specific region of the primary motor cortex.

Now let’s get back to the man with six arms. In his cortical homunculus we will be able to recognize each single arm region. This is due to the fact that, in his case, a cortical reorganization takes place in order to best define its personal “body within the brain” representation, including its peculiarities. Cortical reorganization is somehow continuous during the whole life and corresponds to the redistribution of nerves in those areas that undergo some changes, becoming different than before. This process is not immediate and takes some time. In some cases, as we are going to see, some problems may arise.

Amputees may suffer from the Phantom Limb Syndrome. What’s that? Below here we have a nice definition, found on this website:

Phantom limb syndrome is the perception of sensations, usually including pain, in a limb that has been amputated. Patients with this condition experience the limb as if it were still attached to their body, as the brain continues to receive messages from nerves that originally carried impulses from the missing limb. The exact cause of phantom limb syndrome is unknown. Presumably, the sensations are due to the brain’s attempt to reorganize sensory information following the amputation. The brain must essentially “rewire itself” to adjust to the body change.

Approximately, 60 to 80% of amputees suffer from this syndrome. They might feel the sensation of having the missing limb shorter or longer than normal, or positioned in a wrong and painful way, or they may experience tingling, cramping, heat, and cold sensations in the portion of the limb that was removed. Why does this happen? Even if the limb is no longer part of the body, the nerve endings at the amputation site continue to send signals to the brain. At the level of the primary motor cortex, the brain “thinks” the limb is still there! At the same time, slowly, cortical reorganization starts “updating” the distribution of cortex regions.

A more annoying side of Phantom Limb Syndrome is that of Phantom Limb Pain. It is the arising of mild to extreme pain felt in the amputation area. In general, any sensation that the limb could have experienced prior to the amputation may be experienced again and amplify pain conditions. Usually, corresponding to cortical reorganization, phantom limb sensations usually disappear or decrease over time. Some medical treatments usually include heat application, relaxation techniques (massage, electrical nerve stimulation), neurostimulation techniques (such as spinal cord or deep brain stimulation) or surgery (in the case of scar tissues entangling a nerve).

sources: uno, due e tre