Do you mean a pilonidal sinus or pilonidal cyst?
What I like about wound vacs is that they are kind of like arts and crafts. You can use lots of different techniques and tricks to help achieve a good seal.
The most important thing is going to be the specific size and location of the wound. A wound that is within the crease of the buttocks is going to be more difficult to achieve a seal due to the shape of the skin folds and the fact that that area becomes moist and is manipulated frequently. Here's some basis tips/tricks I suggest trying.
1. During the dressing change, position the patient correctly. for a pilonidal cyst, this probably means putting the patient prone, if at all possible. If not, have them turned as much to the side as possible to you can get a good visualization of the area.
2. Cleanse and thoroughly dry the area before redressing. Dirt, sweat, hairs and moisture will interfere with your seal. This isn't a time to rush things. Make sure you spread the buttocks to allow the creases to dry as well. The creases will be your most difficult areas.
3. Use skin prep with an adhesive component after you have reapplied the foam, but before you put on the transparent dressing. This will help your transparent dressing adhere to the skin. Again, this needs to thoroughly dry. Again, spread the buttocks when applying the skin prep and keep them spread until it's dry. You'll ideally need two people for this. Waiting to apply the skin prep until after you have placed the foam reduces the chances that the skin will become dirty (dropped foam particles) or moist (sweat) which will hinder your seal.
4. When you apply transparent dressing, spread the skin so that it's taut. Place the dressing starting in the creased areas. So if you're working with a cyst in the creases of the buttock, spread the buttock, apply skin prep, stay spread until it's dry and have someone hold the buttock spread while you place the transparent dressing into the crease. Then carefully press the dressing down, starting the in the middle of the crease and working your way to the outer sides of the buttock. If you do it the other way- starting superior to the buttock and then trying to apply your dressing into the crease- you'll end up with wrinkles in the dressing in the creases. Likewise, if you don't spread the skin taut before you put on the dressing you won't have a secure seal.
5. Check your seal when you're done applying the transparent dressing by turning on the vac. If you have a leak, try to isolate it and reinforce the area with an application of skin prep and other piece of transparent dressing.
6. You might find it helpful to put a piece of duoderm (cut it into a crescent like shape) on the inferior portion of the dressing, superior to the anus. This can help keep stool from reaching the dressing as well as protect the inferior edge of the dressing when the patient is wiping themselves. Duoderm is thicker than the transparent dressing, so the patient should be able to feel the edge of the duoderm when wiping- reminding them to be careful in that area.
* The transparent dressing is what creates your seal when using the wound VAC. The duoderm can be used around the dressing or even under the dressing (but under the dressing it needs to be completely covered with transparent dressing), but it shouldn't be used to overlap the edges of the transparent dressing. Duoderm won't stick as well as the transparent dressing- it gets peeled more easily- and it will cause the edges of the dressing to peel. Your best seal will be achieved with clean skin, skin prep, and properly applied transparent dressing. I've never used gel adhesive on a wound VAC, so I'm not sure how it would be of benefit.*
7. Finally, education is key. Try to teach the patient to avoid scratching, wearing tight clothing, or scrubbing the area. Remind them to use caution when on the toilet and cleaning themselves. They need to try to keep the area as dry as possible. If this is an inpatient, staff should be educated that the patient has a VAC (post a sign at the head of the bed) and taught about how to appropriately care for and monitor the area. If the patient is immobile, they should be positioned on the sides or prone. But care needs to be taken when they are being moved in bed and when peri-care is performed. Staff should avoid pulling or dragging the patient along the sheets. Allow some air to get to the area frequently to avoid moisture build up. Make sure they allow the patient's peri area to fully dry before replacing the brief or underwear for the same reason.
If you can describe more thoroughly the type, size and location of the wound as well as where you are having trouble, I could try to provide more specifics.