Patent Publication Number: US-2020297488-A1

Title: A prosthetic device for mitral valve repair

Description:
TECHNICAL FIELD 
     The present application relates to a prosthetic for mitral valve repair, methods of use and manufacture of a prosthetic for mitral valve repair in a human or animal heart. 
     BACKGROUND TO THE INVENTION AND PRIOR ART 
     The mitral valve (also commonly known as the bicuspid valve or left atrioventricular valve) is one of the four valves that control blood flow within the human heart. The mitral valve is the valve which controls blood flow between the left atrium and the left ventricle of the heart. As the blood pressure increases within the left atrium (from blood arriving in the pulmonary veins), the mitral valve opens to allow blood to flow from the left atrium into the left ventricle. Subsequently, the mitral valve closes upon contraction of the heart to prevent blood flowing back into the left atrium. Hence, the mitral valve is commonly known as the “in-flow valve” of the heart. 
     As can be imagined, malfunction of the mitral valve can have serious health effects. There are many well-known causes of mitral valve malfunction, including congenital conditions, degenerative conditions and disease. The background causes of malfunction of the mitral valve, in so far as they are understood, are known in the art, and so are not explained in detail in the present disclosure. The present invention relates to methods and apparatus for rectification of the defects in the mitral valve once they have been detected. 
       FIG. 1  provides a schematic illustration of a typical human heart. Two of the heart&#39;s valves are illustrated in  FIG. 1 , the mitral valve  100  and the tricuspid valve  150 . In the present disclosure, only the mitral valve  100  will be further discussed. The mitral valve  100  is made up of two “flaps”  110 ,  120  which, when the valve  100  is closed, closely align to prevent blood flow through the valve. A healthy valve  110  has an approximately 3:4 anteroposterior diameter  130  to transverse diameter  140  ratio during systole (the contraction phase of the cardiac cycle). I.e. mitral heart valves  110  are generally annular, or “kidney-shaped”, valves.  FIGS. 2 and 3  are respective cross-section representations of healthy  200  and un-healthy  300  human mitral valves during systole. The mitral valve  200  shown in  FIG. 2  has the correct geometric ratio and hence the two flaps  210  and  220  closely align during systole. The mitral valve  200  is made up of two flaps known as the anterior leaflet  210  and the posterior leaflet  220 . The boundaries between the anterior leaflet  210  and the posterior leaflet  220  are marked by the anterolateral commissure line  230  and the posteromedial commissure line  240 . 
     The mitral valve  200  is typically further divided, according to Carpentier&#39;s surgical classification, as follows. The mitral valve posterior leaflet  220  is divided into three scallops, the anterior scallop  221 , the middle scallop  222  and the posterior scallop  223 . The mitral valve anterior leaflet  210  is divided into three corresponding segments, the anterior segment  211 , the middle segment  212  and the posterior segment  213 . Finally, two further segments are defined on the mitral valve anterior leaflet  210 , the anterior commissure  270  and the posterior commissure  280 . 
     In the healthy mitral valve  200 , it can be seen that corresponding scallops ( 221 ,  222 ,  223 ) and segments ( 211 ,  212 ,  213 ) closely align during systole, forming a seal.  FIG. 3  shows a mitral valve with an abnormal geometric ratio, wherein it can be seen that corresponding scallops ( 321 ,  322 ,  323 ) and segments ( 311 ,  312 ,  313 ) no longer align, leaving gap  390 . 
     The mitral heart valve shown in  FIG. 3  has an abnormal geometric ratio, causing the two flaps  310 ,  320  fail to align properly. This can lead to an insufficient seal of the mitral valve  300 , which in turn leads to blood regurgitation and reverse blood flow. 
     Mitral heart valves  300  with abnormal geometric ratios are commonly fixed by the suturing of a prosthetic ring to the mitral valve which remodels its shape, by altering the anteroposterior and transverse diameters, back to the geometric ratio of a normally functioning valve. In cases where these prosthetic rings have been surgically installed, normal blood flow through the heart and the pumping function of the heart can usually be restored. 
     The surgical techniques necessary to install mitral valve prosthetics are collectively known in the art as mitral valve annuloplasty. Generally, these techniques involve isolating the heart from the circulatory system, accessing the left atrium, aligning the mitral valve prosthetic and then stitching (fixing) the mitral valve prosthetic onto the mitral valve to effect a permanent re-orientation of the valve geometric ratio. 
     The use of mitral valve prosthetic rings is generally considered to have been pioneered by the French heart surgeon Alain Carpentier. Since the creation and use of the first mitral valve prosthetic in the 1980s many designs of mitral valve prosthetics have been created and used by surgeons, some of which are shown in  FIGS. 4A to 4F , as will be described in more detail in the following sections. 
     Whilst suitable surgical techniques have been established, considerable debate remains over the best form of mitral valve prosthetic to use, in particular due to the natural variation in anatomy throughout the population. Prior art mitral valve prosthetics generally conform to the annular kidney shape of the mitral valve they are designed to repair. 
     As can be appreciated from  FIGS. 4A to 4F , differences in the exact shape are known in the prior art. However, fundamentally, the shapes of mitral valve prosthetics are restricted to those that conform, at least partially, to the shape of a mitral heart valve. As shown in  FIGS. 4A to 4F , many different form factors of mitral valve prosthetic have been developed. However, there has been a consistent belief that the posterior leaflet must in all cases be supported around its outer edge, by any prosthesis used in the prior art. Despite the availability of an extensive body of optional arrangements of mitral valve prosthetics, post-surgical complications can still be experienced by patients who have undergone mitral valve repair. 
     A typical mitral valve prosthetic  500  is shown in  FIG. 5 . The prosthetic  500  can be divided into four sections. The prosthetic  500  has a substantially straight base portion  510 , a central curved portion  520  located opposite the base portion  510 , and two lateral curved portions  530 ,  540  located between the base portion  510  and the curved portion  550 . Thus, the prosthetic  500  forms a substantially annular “kidney shaped” form. 
       FIG. 6 a    shows a cross section of a human heart  600  in which a mitral valve  690  has been repaired by the fixation of a prior art mitral valve prosthetic  650 . In  FIG. 6 a    the mitral valve has been marked where possible in accordance with the convention of  FIG. 2 , that is the anterior leaflet  610 , the posterior leaflet  620  are indicated. 
     The mitral valve prosthetic  650  has previously been defined as divided into four sections. The mitral valve prosthetic  650  has a substantially straight base portion  651 , a central curved portion  652  located opposite the base portion  651 , and two lateral curved portions  653 ,  654  located between the base portion  651  and the curved portion  652 . 
       FIG. 6 a    illustrates the close shape alignment of the mitral valve prosthetic  650  to the mitral valve. Consequently, shape of the mitral valve prosthetic  650  can be further defined, for ease of reference, in relation to the anatomical definitions of the mitral valve. The substantially straight base portion  651  closely aligns to the aortic mitral curtain. The central curved portion  652  and two lateral curved portions  653 ,  654  closely align with the perimeter of the mitral valve posterior leaflet  620 . 
     The mitral valve  690  and the mitral valve prosthetic  650  can be further defined.  FIG. 6 b    shows an enlarged version of mitral valve  690  and the mitral valve prosthetic  650 , in which they further definitions are added. The markings of  FIG. 6 b    should be understood as applicable to all comparable figures in the present application. The mitral valve  690  can be further divided, according to Carpentier&#39;s surgical classification. The mitral valve posterior leaflet  620  is divided into three scallops, the anterior scallop  621 , the middle scallop  622  and the posterior scallop  623 . The mitral valve anterior leaflet  610  is divided into three corresponding segments, the anterior segment  611 , the middle segment  612  and the posterior segment  613 . Finally, two further segments are defined on the mitral valve anterior leaflet  610 , the anterior commissure  670  and the posterior commissure  680 . 
     Moreover, the central curved portion  652  and two lateral curved portions  653 ,  654  of the mitral valve prosthetic can be even further defined in relation to Carpentier&#39;s surgical classification of the mitral valve. The central curved portion  651  and two lateral curved portions  653 ,  654  of the mitral valve prosthetic can be divided into five “zones”  655  to  659 . Zone  655  corresponds to the anterior commissure  670 , zone  656  corresponds to the anterior scallop  621 , zone  657  corresponds to the middle scallop  622 , zone  658  corresponds to the posterior scallop  623 , and zone  659  corresponds to the posterior commissure  680 . 
     Mitral valve repair is generally successful, with some studies estimating that there is up to a 95% chance that no further surgical intervention will be required in the 10 years following a mitral valve annuloplasty (Heart. 2006 February; 92(2): 275-281). However, despite the low re-operation rate of mitral valve annuloplasties, there remains a risk of causing damage to the tissue surrounding the mitral valve prosthesis upon fixation of the device. 
     Despite the low re-operation rate, it is typically expected that approximately 20% of patients will suffer some degree of reduced ventricular function following a mitral valve repair annuloplasty. Of this 20% of patients, up to a third do not fully their pre-operation ventricular function. 
     SUMMARY OF THE INVENTION 
     Other features and advantages of the present invention will be apparent from the appended claims. 
     In a first aspect, the present invention provides a prosthetic ring for repair of a mitral valve, the prosthetic ring forming at least part of a shape having: a substantially straight base portion, configured to align with the aortic mitral curtain; a central curved portion, arranged opposite the base portion, having a first radius of curvature, R 1 ; and a first and second lateral curved portions, located either side of the central curved portion, wherein the first and second lateral curved portions have a second radius of curvature, R 2 ; wherein R 2 &lt;R 1 ; and a gap in the circumference of the ring formed in at least a portion of the first lateral curved portion or the second lateral curved portion. 
     In a second aspect, the present invention provides a prosthetic ring for repair of a mitral valve, having a substantially annular form or a part annular form, wherein the prosthetic ring occupies at least a part of a shape having: a substantially straight base portion, configured to align with the aortic mitral curtain; a central curved portion, arranged opposite the base portion, configured to align with the posterior leaflet of the mitral valve in the region of the middle scallop; a first lateral curved portion, located between the first end of the central curved portion and the first end of the base portion, configured to align with the anterolateral commissure of the mitral valve; a second lateral curved portion, located between the second end of the central curved portion and the second end of the base portion, configured to align with the posteromedial commissure of the mitral valve; and a gap in the circumference of the ring formed in at least a portion of the first lateral curved portion or the second lateral curved portion. 
     The prosthetic ring may further comprise a posterior ring part and an anterior ring part coupled to the posterior ring part at at least one coupling point on a transverse plane defining a maximum width of the prosthetic ring; wherein the prosthetic ring comprises a gap or a removable portion located in the posterior ring part. 
     The gap in the circumference of the prosthetic ring of the present invention may correspond to at least the portion of the prosthetic ring aligned with the anterolateral commissure. The gap in the circumference of the prosthetic ring of the present invention may also correspond to at least the portion of the prosthetic ring aligned with the anterior scallop of the mitral valve. Similarly, the gap of the prosthetic ring of the present invention may correspond to at least the portion of the prosthetic ring aligned with the circumflex branch of the left coronary artery. 
     A portion of the prosthetic ring of the present invention opposite a gap in the prosthetic ring may a) have an increased stiffness, b) have an increased thickness, or c) be formed from a stiffer material, than the rest of the prosthetic ring. Furthermore, the prosthetic ring of the present invention may comprise multiple layers of material. A portion of the prosthetic ring of the present invention opposite a gap in the prosthetic ring may have an increased number of layers of material. A portion of the prosthetic ring of the present invention opposite a gap in the prosthetic ring may be made of a material with a flexural modulus which is higher than the material of the remainder of a circumference of the prosthetic ring. Moreover, the prosthetic ring of the present invention may further comprise a suture permeable coating. 
     In a third aspect, the present invention provides a prosthetic ring for repair of a mitral valve, having a substantially annular form or a part annular form, wherein the prosthetic ring occupies at least a part of a shape having: a substantially straight base portion, configured to align with the aortic mitral curtain; a central curved portion, arranged opposite the base portion, configured to align with the posterior leaflet of the mitral valve in the region of the middle scallop; a first lateral curved portion, located between the first end of the central curved portion and the first end of the base portion, configured to align with the anterolateral commissure of the mitral valve; a second lateral curved portion, located between the second end of the central curved portion and the second end of the base portion, configured to align with the posteromedial commissure of the mitral valve; and wherein the prosthetic ring comprises a cut-able portion, located on the first lateral curved portion or the second lateral curved portion, which when removed, provides a gap in the circumference of the ring formed in the first lateral curved portion or the second lateral curved portion. 
     In a fourth aspect, the present invention provides a method of preparing a prosthetic ring for repair of a mitral valve, comprising: removing a portion of the ring; wherein the prosthetic ring occupies at least a part of a shape having: a substantially straight base portion, configured to align with the aortic mitral curtain; a central curved portion, arranged opposite the base portion, configured to align with the posterior leaflet of the mitral valve in the region of the middle scallop; a first lateral curved portion, located between the first end of the central curved portion and the first end of the base portion, configured to align with the anterolateral commissure of the mitral valve; a second lateral curved portion, located between the second end of the central curved portion and the second end of the base portion, configured to align with the posteromedial commissure of the mitral valve; and wherein removing the portion of the ring provides a gap in the circumference of the ring formed in the first lateral curved portion or the second lateral curved portion. 
     The method of preparing the prosthetic ring may be performed on a prosthetic ring having a substantially annular form or a part annular form, the prosthetic ring further comprising a posterior ring part and an anterior ring part coupled to the posterior ring part at at least one coupling point on a transverse plane defining a maximum width of the prosthetic ring; wherein removing the portion of the ring to provide a gap in the circumference of the ring comprises forming a gap located in the posterior ring part. 
     The method of preparing the prosthetic ring may further comprise forming a gap in at least the portion of the prosthetic ring configured to align with the anterolateral commissure of the mitral valve. The method of preparing the prosthetic ring may further comprise forming a gap in at least the portion of the prosthetic ring configured to align with the anterior scallop of the mitral valve. The method of preparing the prosthetic ring may further comprise forming a gap in at least the portion of the prosthetic ring configured to align with the circumflex branch of the left coronary artery. 
     In a fifth aspect, the present invention provides a prosthetic band for repair of a mitral valve, the prosthetic band forming at least part of a shape having: a central curved portion, having a first radius of curvature, R 1 ; and a first and second lateral curved portions, located either side of the central curved portion, wherein the first and second lateral curved portions have a second radius of curvature, R 2 ; wherein R 2 &lt;R 1 ; and a gap in the band, the gap extending over a, preferably substantially straight, base portion configured to align with the aortic mitral curtain and at least a portion of the first lateral curved portion or the second lateral curved portion. 
     In a sixth aspect, the present invention provides a prosthetic band for repair of a mitral valve, having a substantially part annular form, wherein the prosthetic band occupies at least a part of a shape having: a central curved portion, configured to align with the posterior leaflet of the mitral valve in the region of the middle scallop; a first lateral curved portion, located at the first end of the central curved portion, configured to align with the anterolateral commissure of the mitral valve; a second lateral curved portion, located at the second end of the central curved portion, configured to align with the posteromedial commissure of the mitral valve; and a gap in the band, the gap extending over a, preferably substantially straight, base portion configured to align with the aortic mitral curtain and at least a portion of the first lateral curved portion or the second lateral curved portion. 
     The gap in the prosthetic band may extend over at least the portion of the prosthetic band configured to align with the anterolateral commissure of the mitral valve. The gap in the prosthetic band may also extend over at least the portion of the prosthetic band configured to align with the anterior scallop of the mitral valve. The gap in the prosthetic band may also extend over to at least the portion of the prosthetic band configured to align with the circumflex branch of the left coronary artery. 
     In a seventh aspect, the present invention provides a prosthetic band for repair of a mitral valve, having a substantially part annular form, wherein the prosthetic band occupies at least a part of a shape having: a central curved portion, configured to align with the posterior leaflet of the mitral valve in the region of the middle scallop; a first lateral curved portion, located at the first end of the central curved portion, configured to align with the anterolateral commissure of the mitral valve; a second lateral curved portion, located at the second end of the central curved portion, configured to align with the posteromedial commissure of the mitral valve; and wherein the prosthetic band comprises a cut-able portion, located on the first lateral curved portion or the second lateral curved portion, which when removed, provides a gap in the band extending over the first lateral curved portion or the second lateral curved portion. 
     In an eighth aspect, the present invention provides a method of preparing a prosthetic band for repair of a mitral valve, comprising: removing a portion of the prosthetic band; wherein the prosthetic band occupies at least a part of a shape having: a central curved portion, configured to align with the posterior leaflet of the mitral valve in the region of the middle scallop; a first lateral curved portion, located at the first end of the central curved portion, configured to align with the anterolateral commissure of the mitral valve; a second lateral curved portion, located at the second end of the central curved portion, configured to align with the posteromedial commissure of the mitral valve; and wherein removing the portion of the band provides a gap in the band extending over the first lateral curved portion or the second lateral curved portion. 
     Removing the portion of the band to provide a gap in the band may comprise forming a gap in at least the portion of the prosthetic band configured to align with the anterolateral commissure of the mitral valve. Removing the portion of the band to provide a gap in the band may also comprise forming a gap in at least the portion of the prosthetic band configured to align with the anterior scallop of the mitral valve. Removing the portion of the band to provide a gap in the band may also comprise forming a gap in at least the portion of the prosthetic band configured to align with the circumflex branch of the left coronary artery. 
     In a ninth aspect, the present invention provides a method of repairing a mitral valve using a prosthetic ring or prosthetic band according to any of the first to eight aspects, comprising the steps of: suturing the prosthetic ring, or the prosthetic band, to a mitral valve to remodel its shape. 
     In a tenth aspect, the present invention provides a method of repairing a mitral valve comprising the steps of: preparing a prosthetic ring or prosthetic band according to the method of any of the first to eighth aspects; and suturing the prosthetic ring, or the prosthetic band, to a mitral valve to remodel its shape. 
     It will be evident to the skilled reader that the above definitions of features of the invention relate to same or similar products and uses for such products and so any or all of the above described features of the invention can be separated or combined in various combinations while still realising the benefits of the invention. 
     Other features and advantages of the present invention will be apparent from the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Some embodiments of apparatus and/or methods in accordance with embodiments of the present invention are now described, by way of example only, and with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross-section representation of a human heart 
         FIG. 2  shows a cross-section representation of a healthy human mitral valve during systole; 
         FIG. 3  shows a cross-section representation a misaligning mitral valve during systole; 
         FIGS. 4A to 4F  show plan views of mitral valve prosthetics known from the prior art; 
         FIG. 5  shows a plan view of a mitral valve prosthetic known from the prior art; 
         FIG. 6 a    shows a cross-section representation of mitral valve repaired by fixation of a prior art mitral valve prosthetic; 
         FIG. 6 b    shows an enlarged representation of the repaired mitral valve of  FIG. 6 a   ; 
         FIG. 7  shows a mitral valve prosthetic according to the present invention; 
         FIG. 8  shows a cross-section representation of mitral valve repaired by fixation of a mitral valve prosthetic according to an embodiment of the present invention; 
         FIG. 9 a    shows a mitral valve prosthetic according to the present invention; 
         FIG. 9 b    shows a mitral valve prosthetic according to the present invention; 
         FIG. 10  shows a mitral valve prosthetic according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     To aid the skilled person&#39;s understanding, where appropriate, an imaginary x-axis and y-axis has been included in the figures. Where included, the x-axis is marked “x”, and the y-axis is marked “y”. 
     The applicant has identified that 3% to 5% of patients will suffer some degree of circumflex artery distortion or injury, during a mitral valve annuloplasty. Circumflex artery damage may lead to a myocardial infarction (a heart attack), generally due to unrecoverable left ventricular infarction, unless quickly diagnosed and treated. 
     One difficulty faced by surgeons performing mitral heart valve annuloplasties is that the surgeon may not know beforehand the relative location of the heart vasculature to the operation site. The surface of the heart is covered by a complicated vasculature system, as can be seen, for example, in  FIG. 6 a   . 
     The applicant has also identified that a significant potential cause of complications during or after mitral valve annuloplasty can be the surgical impingement of the vasculature in proximity to the mitral valve. These problems are particularly acute in patients who, through natural anatomical variation, are considered to have a higher risk anatomy. In particular, the applicant has identified that patients with heart vasculature in which the circumflex branch of the coronary artery lies close to the mitral valve are particularly at risk of operative complications during and following a mitral valve annuloplasty. Therefore, there exists a need for an improved mitral valve prosthetic which mitigates some of the causes of complications experienced in mitral valve annuloplasty procedures for those patients where natural variation has caused particularly high risk location of the vasculature in proximity of the mitral valve. Particularly, it has been identified that certain patients can be identified as at a high risk of operative circumflex artery damage. 
     A first embodiment of the invention will now be described with reference to  FIG. 7 .  FIG. 7  shows a mitral valve prosthetic  700 . The shape of the prosthetic  700  conforms to an annular, kidney-shaped form. Thus, the shape of the prosthetic  700  can be divided into four sections. The shape of prosthetic  700  has a substantially straight base portion  710 , a central curved portion  720  located opposite the base portion  710 , and two lateral curved portions  730 ,  740  located between the base portion  710  and the curved portion  720 . 
     Mitral valve prosthetic  700  may be made of a resilient bio-compatible material. The resilience of the material allows the prosthetic  700  to be at least partially deformed during surgery and provides a force to the mitral valve which restores the natural anteroposterior diameter to transverse diameter ratio geometric ratio. Bio-compatibility ensures that the prosthetic  700  does not promote blood clotting or bacterial growth. Many suitable resilient bio-compatible materials are known in the art, including bio-compatible polymers, metals and metal-alloys. Preferably, at least part of the prosthetic  700  is made of titanium or a titanium alloy. 
     The mitral valve prosthetic  700  may be covered in suture-permeable covering  750 . The suture-permeable covering can aid the surgeon in fixing the prosthetic  700  to the mitral valve. Suture-permeable coverings for prosthetics are well known in the art and the prosthetic  700  may use any such suitable covering. Preferably, the suture-permeable covering is Dacron® (polyethylene terephthalate) coated silicon. 
     The mitral valve prosthetic  700  has a gap  760  in its circumference. The gap  760  corresponds to the region of the mitral valve close to the circumflex artery and represents either a manufactured gap or removed portion in the circumference of the prosthetic. Since gap  760  is provided in the prosthetic  700 , in use, no sutures can be placed within the region of the mitral valve closest to the circumflex artery. This reduces the likelihood of circumflex artery impingement or damage. This goes against the strongly held prejudice in the art to provide as much support as possible to the circumference of the mitral valve; or in cases where the anterior leaflet of the mitral valve is unaffected by the underlying pathology, to provide as much support as possible to at least the entirety of the posterior leaflet of the mitral valve. Evidence of this prejudice in the art can be seen, as discussed previously, in prior art prostheses such as those shown in  FIGS. 4A to 4F . 
       FIG. 8  illustrates the mitral valve prosthetic  850  once it has been fixed to a mitral valve  890 . As can be seen from  FIG. 8 , parts of the circumflex artery  891  (otherwise known as the circumflex branch of the coronary artery) are located close to the mitral valve  890 . Therefore, the gap  760 ,  860  corresponds to, at least, the anterior commissure  670 ,  870 , i.e. zone  655 ,  855 . Furthermore,  FIG. 8  also illustrates that the region of the circumflex artery  891  in close proximity to the mitral valve  890  may extend from the region of the anterior commissure  670 ,  870  to at least partially surround the region of the anterior scallop  621 ,  821 . Consequently, the gap  760 ,  860  may also correspond to, at least part of, the anterior commissure  670 ,  870  and the anterior scallop  621 ,  821  i.e. zone  655 ,  855  and zone  656 ,  856 . 
     As will be appreciated, the gap  760 ,  860  in the mitral valve prosthetic  700  will reduce the overall stiffness of the mitral valve prosthetic  700 . To compensate, the mitral valve prosthetic preferably comprises a material with a stiffness higher than a prior art mitral valve prosthetic. Increased stiffness can be achieved by any suitable method of stiffening an object, including using a material with a higher stiffness, combining multiple layers of material, thickening at least a portion of the ring and/or incorporating multiple materials of differing stiffnesses into the ring. This is preferably implemented in an area substantially opposite or adjacent to gap  760 ,  860 . 
     The applicant recognises that omitting or removing a portion of the mitral valve prosthetic  700  may decrease the durability of the prosthetic  700  due to increased flexing under contraction of the heart. This is a further reason for which the configurations of the present invention have been avoided in the prior art. However, in conceiving the invention, it has been recognised that clinically, the potentially lower durability of the prosthetic  700  may be preferable to the risk of damage to heart vasculature through infarction of the lateral wall of the heart for certain patients due to damage caused to vasculature in close proximity to the area of suturing. 
     In  FIG. 7 , mitral valve prosthetic  700  is shown as being substantially symmetrical along a central midline. The skilled person will understand that the shape of mitral valve prosthetic  700  can deviate from being perfectly symmetrical whilst still conforming to the shape of the mitral valve. In fact, the mitral valve prosthetic must be at least partially deformable in order to be installed by common key-hole surgical methods. Furthermore, the mitral valve prosthetic is sutured onto a mitral valve which continuously deforms as the heart beats. Thus, in use, the mitral valve prosthetic  700  must resiliently correct the shape of the mitral valve whilst allowing for the natural cycle of movement of the valve. 
       FIG. 7  also illustrates only a 2-dimensional representation of mitral valve prosthetic  700 . This should not be taken to mean that mitral valve prosthetic  700  is flat in the third dimension (i.e. a z-axis which is normal to both the marked x-axis and y-axis). In fact, mitral valve prosthetic  700  may be formed to have a substantially curved profile along the z-axis, as is used in some prior art mitral valve prosthetics. 
     Mitral valves come in many different sizes and consequently, the mitral valve prosthetic  700  may be supplied in a series of differing sizes, all conforming to the same shape of the claimed invention, to fit differing sizes of mitral valves. The larger mitral valve prosthetics  700  may require increased stiffening in order to sufficiently correct the abnormal geometric ratio of the damaged mitral valves. This increased stiffening may be performed by any of the methods described above. Conversely, the smaller sizes of mitral valve prosthetic may require less stiffening in order to adequately correct the mitral valve. 
     Whilst mitral valve prosthetics with removed sections are known from the prior art, generally termed “mitral bands” (see for example  FIG. 4D  of the present application), all of these prior art mitral valve prosthetics define a removed or removable section within the substantially straight base portion ( 710 ) of the mitral valve prosthetic; i.e. the prior art mitral valves with a gap are effectively “C” shaped. These “C” shaped mitral valves are so shaped in order to preserve the natural shape and function of the anterior leaflet of the mitral valve, whilst providing full support around the circumference of the posterior leaflet. Consequently, these prior art mitral valve prosthetics should only be used where the anterior leaflet of the mitral valve is unaffected by the underlying pathology requiring treatment. Thus, these prior art mitral valve prosthetics are aimed at solving a different technical problem to the presently claimed invention, that of providing a mitral valve prosthetic which does not alter or restrict the shape and movement of the anterior leaflet of the mitral valve. Differently to the present invention, all prior art mitral valve prosthetics have until now provided support to the whole of the posterior leaflet of the mitral valve. 
     Further embodiments will now be further described in relation to  FIG. 9 a   . The mitral valve prosthetic  900  conforms to mitral valve  700 , shown in  FIG. 7 , but is defined geometrically rather than by reference to anatomical features. Mitral valve prosthetics are commonly described geometrically in the prior art, as can be seen in EP 1258232 A2 and U.S. 2011/0224786 A1. Mitral valve  900  is thus described, where possible, using equivalent geometric terms as those used in the prior art. 
     Mitral valve prosthetic  900  comprises an anterior half-ring  910  which is substantially straight, and a curved posterior half-ring  920 . The posterior half-ring  920  and anterior half-ring  910  are coupled at one point  950  located substantially on a lateral plane  930  that defines a maximum width section of the prosthesis. In addition, a longitudinal plane  940  is also defined, that intersects the prosthesis substantially at points  970  and  980 , that is arranged substantially perpendicular to the lateral plane  930  and substantially bisects the anterior half-ring  950  at point  980 . The posterior half-ring  920  is thus subdivided in to a first lateral zone (left)  990  located between the points  950  and  970 , and a second lateral zone (right)  995  located between the points  965  and  970 . 
     The intersection points  950 ,  970  and  980  of the prosthesis respectively with the planes  930  and  940  define geometric points used for the calculation of the dimensions of the prosthesis. In accordance with the prior art, the distance between the points  970  and  980 , defines the height of the prosthesis. The length of the anterior half-ring  910 , defines a width of the prosthesis. Typically, the ratio of the height of the mitral valve prosthesis to the width of the prosthesis is approximately 3:4. 
     Mitral valve prosthetic  900  is made of a resilient bio-compatible material. The resilience of the material allows the prosthetic  900  to be at least partially deformed during surgery and provides a force to the mitral valve which restores the natural anteroposterior diameter to transverse diameter ratio geometric ratio. Bio-compatibility ensures that the prosthetic  900  does not promote blood clotting or bacterial growth. Many suitable resilient bio-compatible materials are known in the art, including bio-compatible polymers, metals and metal-alloys. Preferably, at least part of the prosthetic  900  is made of titanium or a titanium alloy. 
     The mitral valve prosthetic  900  is covered in suture-permeable covering  955 . The suture-permeable covering aids the surgeon in fixing the prosthetic  900  to the mitral valve. Suture-permeable coverings for prosthetics are well known in the art and the prosthetic  900  may use any such suitable covering. Preferably, the suture-permeable covering is Dacron® (polyethylene terephthalate) coated silicon. 
     The mitral valve prosthetic  900  has a gap  960 . The gap  960  is provided in, or omitted from, at least a portion of the first or second lateral curved portions  990  or  995 . Since portion  960  is removed from the prosthetic  900 , in use, no sutures can be placed within these regions. This portion  960  corresponds to the region of the mitral valve closest to the circumflex artery, thus reducing the likelihood of circumflex artery impingement or damage. 
     As will be appreciated, removing a portion  960  of the mitral valve prosthetic  900  will reduce the overall stiffness of the mitral valve prosthetic  900 . To compensate, the mitral valve prosthetic is preferably made of a material with a stiffness higher than a prior art mitral valve prosthetic. Increased stiffness can be achieved by any prior art method of stiffening an object, including using a material with a higher stiffness, combining multiple layers of material, thickening at least a portion of the ring and/or incorporating multiple materials of differing stiffnesses into the ring. This is preferably implemented in an area substantially opposite or adjacent to gap  960 . 
     The applicant recognises that omitting or removing a portion of the mitral valve prosthetic  900  may decrease the durability of the prosthetic  900  due to increased flexing under contraction of the heart. However, it has been recognised that clinically, the potentially lower durability of the prosthetic  900  may be preferable to the risk of damage to heart vasculature through infarction of the lateral wall for certain patients. 
     In  FIG. 9 a   , mitral valve prosthetic  900  is shown as symmetrical along a central midline. The skilled person will understand that the shape of mitral valve prosthetic  900  can deviate from being perfectly symmetrical whilst still conforming to the shape of the mitral valve. In fact, the mitral valve prosthetic must be at least partially deformable in order to be installed by common key-hole surgical methods. Furthermore, the mitral valve prosthetic is sutured onto a mitral valve which continuously deforms as the heart beats. Thus, in use, the mitral valve prosthetic  900  must resiliently correct the shape of the mitral valve whilst allowing for the natural cycle of movement of the valve. 
     Mitral valves come in many different sizes and The mitral valve prosthetic  900  may be supplied in a series of differing sizes, all conforming to the same shape of the claimed invention, to fit differing sizes of mitral valves. The larger mitral valve prosthetics  900  may require increased stiffening in order to sufficiently correct the abnormal geometric ratio of the damaged mitral valves. This increased stiffening may be performed by any of the methods described above. 
     Further embodiments will now be described in relation to  FIG. 9 b   .  FIG. 9 b    illustrates the mitral valve prosthetic  900  of  FIG. 9 a    in a prosthetic band configuration. As has been described previously, prosthetic bands (or “mitral bands”) are known in the art. These known prosthetic bands can comprise a removed or removable section within the previously defined substantially straight base portion ( 710 ) of the mitral valve prosthetic. These known prosthetic bands are shaped in order to preserve the natural shape and function of the anterior leaflet of the mitral valve. Therefore, these prosthetic bands are preferably only used when the anterior leaflet of the mitral valve is functionally normal and does not require further support. 
     There are patients who would otherwise be suitable for a prosthetic band, but have an anatomy which gives rise to a high risk of surgical impingement of the vasculature in proximity to the mitral valve. Hence, they are not suitable for traditional prosthetic bands. As will be appreciated from the discussion above, the presently described novel features of the invention can be equally applicable to a prosthetic band arrangement. For illustration, an example is shown in  FIG. 9 b   . Thus,  FIG. 9 b    shows a prosthetic band, in accordance with embodiments of the present invention, which are particularly suitable for patients with high risk anatomies, but a fully functioning anterior leaflet of the mitral valve. 
     As described earlier in relation to mitral bands, in  FIG. 9 b   , mitral valve prosthetic  900 ′ does not comprise an anterior half-ring  910 , but otherwise corresponds in shape to the mitral valve prosthetic  900  of  FIG. 9 a   . That is, in  FIG. 9 b   , mitral valve prosthetic  900 ′ comprises a curved posterior half-ring  920 ′.  FIG. 9 b    also illustrates that mitral valve prosthetic  900 ′ comprises a gap  960 ′ which extends over the base portion (not shown), i.e. the missing anterior half-ring  910  of  FIG. 9   a.    
     As with the example of  FIG. 9 a   , the mitral valve prosthetic  900 ′ of  FIG. 9 b    is also made of a resilient bio-compatible material. The resilience of the material allows the prosthetic  900 ′ to be at least partially deformed during surgery and provides a force to the mitral valve which restores the natural anteroposterior diameter to transverse diameter ratio geometric ratio. Bio-compatibility ensures that the prosthetic  900 ′ does not promote blood clotting or bacterial growth. Many suitable resilient bio-compatible materials are known in the art, including bio- compatible polymers, metals and metal-alloys. Preferably, at least part of the prosthetic  900 ′ is made of titanium or a titanium alloy. 
     As with the example of  FIG. 9 a   , the mitral valve prosthetic  900 ′ of  FIG. 9 b    is also covered in suture-permeable covering (not shown). The suture-permeable covering aids the surgeon in fixing the prosthetic  900 ′ to the mitral valve. Suture-permeable coverings for prosthetics are well known in the art and the prosthetic  900 ′ may use any such suitable covering. Preferably, the suture-permeable covering is Dacron® (polyethylene terephthalate) coated silicon. 
     As with the example of  FIG. 9 a   , the mitral valve prosthetic  900 ′ has a gap  960 ′. The gap  960 ′ is provided in, or omitted from, at least a portion of the first or second lateral curved portions  990 ′ or  995 ′. Since portion  960 ′ is removed from the prosthetic  900 ′, in use, no sutures can be placed within these regions. This portion  960 ′ corresponds to the region of the mitral valve closest to the circumflex artery, thus reducing the likelihood of circumflex artery impingement or damage. 
     As will be appreciated, the removed portion  960 ′ of the mitral valve prosthetic  900 ′ of  FIG. 9 b    will further reduce the overall stiffness of the mitral valve prosthetic  900 ′. To compensate, the mitral valve prosthetic is preferably made of a material with a stiffness higher than a prior art mitral valve prosthetic. Increased stiffness can be achieved by any prior art method of stiffening an object, including using a material with a higher stiffness, combining multiple layers of material, thickening at least a portion of the ring and/or incorporating multiple materials of differing stiffnesses into the ring. This is preferably implemented in an area substantially opposite or adjacent to gap  960 ′ or the overall gap between ends of the band. 
     The applicant recognises that omitting or removing a greater portion of the mitral valve prosthetic  900 ′ may decrease the durability of the prosthetic  900 ′ due to increased flexing under contraction of the heart. However, it has been recognised that clinically, the potentially lower durability of the prosthetic  900 ′ may be preferable to the risk of damage to heart vasculature through infarction of the lateral wall for certain patients. 
     In  FIG. 9 b   , mitral valve prosthetic  900 ′ is shown as substantially symmetrical along a central midline. The skilled person will understand that the shape of mitral valve prosthetic  900 ′ can deviate from being perfectly symmetrical whilst still conforming to the shape of the mitral valve. In fact, the mitral valve prosthetic must be at least partially deformable in order to be installed by common key-hole surgical methods. Furthermore, the mitral valve prosthetic is sutured onto a mitral valve which continuously deforms as the heart beats. Thus, in use, the mitral valve prosthetic  900 ′ must resiliently correct the shape of the mitral valve whilst allowing for the natural cycle of movement of the valve. 
     Mitral valves come in many different sizes and the mitral valve prosthetic  900 ′ may be supplied in a series of differing sizes, all conforming to the same shape of the claimed invention, to fit differing sizes of mitral valves. The larger mitral valve prosthetics  900 ′ may require increased stiffening in order to sufficiently correct the abnormal geometric ratio of the damaged mitral valves. This increased stiffening may be performed by any of the methods described above. 
     In a second embodiment of the present invention, with reference to  FIG. 10 , there is provided a mitral valve prosthetic  1000 . The shape of the prosthetic  1000  conforms to the annular, kidney-shaped form of the prosthetic  700 ,  800  of the first embodiment. That is, the shape of the prosthetic  1000  can be divided into four sections; a substantially straight base portion  1010 , a central curved portion  1020  located opposite the base portion  1010 , and two lateral curved portions  1030 ,  1040  located between the base portion  1010  and the curved portion  1020 . Alternatively, the shape of the prosthetic  1000  could be considered to comprise a posterior half-ring  1080  and an anterior half-ring  1070  which is coupled to the posterior half ring at at least one coupling point on a first lateral plane. 
     Mitral valve prosthetic  1000  may also be made of a resilient bio-compatible material. The resilience of the material allows the prosthetic  1000  to be at least partially deformed during surgery and provides a force to the mitral valve which restores the natural anteroposterior diameter to transverse diameter ratio geometric ratio. Bio-compatibility ensures that the prosthetic  1000  does not promote blood clotting or bacterial growth. Many suitable resilient bio-compatible materials are known in the art, including bio-compatible polymers, metals and metal-alloys. Preferably, at least part of the prosthetic  1000  is made of titanium or a titanium alloy. 
     The mitral valve prosthetic  1000  may also be covered in suture-permeable covering  1050 . The suture-permeable covering can aid the surgeon in fixing the prosthetic  1000  to the mitral valve. Suture-permeable coverings for prosthetics are well known in the art and the prosthetic  1000  may use any such suitable covering. Preferably, the suture-permeable covering is Dacron® (polyethylene terephthalate) coated silicon. 
     The mitral valve prosthetic  1000  has a removable portion  1060 . The removable portion  1060  corresponds to the region of the mitral valve close to the circumflex artery. The removable portion  1060  is weaker, and hence more easily cut-able, than the rest of the mitral valve prosthetic. I.e. the material making up the removable portion  1060  is less resistant to shear stresses than the material making up the rest of the mitral valve prosthetic. Any suitable method of providing a material less resistant to shear stress may be used, for example, using a different material in the removable portion  1060 , using less layers of material in the removable portion  1060  and/or using thinner layers of material in the removable portion  1060 . 
     The removable portion  1060  may be removed in use by the surgeon to provide a gap  1060  corresponding to the region of the mitral valve close to the circumflex artery. The removable portion, may be made in any portion of the posterior half-ring. The removable portion  1060  may be removed in its entirety or partially. When portion  1060  is removed from the prosthetic  1000 , in use, no sutures can be placed within the region of the mitral valve closest to the circumflex artery, thus reducing the likelihood of circumflex artery impingement or damage. The removable portion  1060  may be made of a different material to the rest of the mitral valve prosthetic. Preferably, the removable portion  1060  is made of a material that is less tough than the rest of the ring, to aid the removal of the removable portion. The removable portion  1060  may further be segmented to aid its partial removal. 
     The suture-permeable covering  1050  may be marked to illustrate the extent of the removable portion  1060 . Any bio-compatible marking may be used, such as changing the colouration of the suture-permeable covering, the addition of further bio-compatible threads. 
     As will be appreciated, removing a portion  1060  of the mitral valve prosthetic  1000  will reduce the overall stiffness of the mitral valve prosthetic  1000 . To compensate, the mitral valve prosthetic preferably comprises a material with a stiffness higher than a prior art mitral valve prosthetic. Increased stiffness can be achieved by any prior art method of stiffening an object, including using a material with a higher stiffness, combining multiple layers of material, thickening at least a portion of the ring and/or incorporating multiple materials of differing stiffnesses into the ring. 
     The applicant recognises that removing a portion of the mitral valve prosthetic  1000  may decrease the durability of the prosthetic  1000  due to increased flexing under contraction of the heart. However, in conceiving the inventions, it has been recognised that clinically, the potentially lower durability of the prosthetic  1000  may be preferable to the risk of damage to heart vasculature through infarction of the lateral wall of the heart for some patients. 
     In  FIG. 10 , mitral valve prosthetic  1000  is shown as being substantially symmetrical along a central midline. The skilled person will understand that the shape of mitral valve prosthetic  1000  can deviate from being perfectly symmetrical whilst still conforming to the shape of the mitral valve. In fact, the mitral valve prosthetic must be at least partially deformable in order to be installed by common key-hole surgical methods. Furthermore, the mitral valve prosthetic is sutured onto a mitral valve which continuously deforms as the heart beats. Thus, in use, the mitral valve prosthetic  1000  must resiliently correct the shape of the mitral valve whilst allowing for the natural cycle of movement of the valve. 
       FIG. 10  also illustrates only a 2-dimensional representation of mitral valve prosthetic  700 . This should not be taken to mean that mitral valve prosthetic  1000  is flat in the third dimension (z-axis). In fact, mitral valve prosthetic  1000  may be formed substantially curved along the z-axis, as is used in some prior art mitral valve prosthetics. 
     The removable portion  1060  can also be applied to the prosthetic band-type embodiment of  FIG. 9 b   , to provide a removable portion at the location of the gap or omitted portion  960 ′. As can be seen from  FIG. 9 b   , mitral valve prosthetic  900 ′ already comprises a gap  960 ′ which extends over the base portion (not shown), i.e. the missing anterior half-ring  910  of  FIG. 9 a   . The removable portion  1060  extends gap  960 ′ over a portion of the first or second lateral curved portions  990 ′ or  995 ′. 
     Mitral valves come in many different sizes and consequently, the mitral valve prosthetic  1000  may be supplied in a series of different sizes, all conforming to the same shape of the claimed invention, to fit differing sizes of mitral valves. The larger mitral valve prosthetics  1000  may require increased stiffening in order to sufficiently correct the abnormal geometric ratio of the damaged mitral valves. This increased stiffening may be performed by any of the methods described above. Conversely, the smaller sizes of mitral valve prosthetic may require less stiffening in order to adequately correct the mitral valve.